Rated by buyers 
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Honestly, when it comes down to it, every single RPG book is a book of houserules. This book lacks a coherent theme, but instead just presents you with a ton of different systems that you can add to your game a la carte. Many of them are very good ideas.
You aren't going to use them all. That's not the point. But you are going to like some, hate others and be inspired by a few more. This is a great book for getting ideas or trying things differently.
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Unearthed Arcana opens with these words and no phrase has ever been more appropriate. This is not a standard core rule book, where the DM can just say yay or nay on whether or not they're using the material. Unearthed Arcana is not even trying to be that kind of book. Unearthed Arcana is a collection of dozens (hundreds, if you count like a marketer) optional rules, many of which conflict with one another. This book has something for everybody and everything for nobody.
If you buy this book thinking your getting another normal core rule book, you're probably not going to like it. If you buy this book hoping to find individual rules to liven your campaign, you'll probably like it very much.
Chapter 1 contains racial rules. These rules are the most niche oriented of the book. For instance, the very first section involves environmental racial variants (like desert elves). Honestly, I don't think I'll ever use these unless I do an extremely theme oriented campaign (like a home grown Dark Sun). This is followed by elemental variants, which are even more niche-like (air elves). Then come bloodlines, which are fairly interesting, if you allow this sort of thing. Bloodlines allow you to introduce racial traits without saying a character is half something. If great great grandmama had strange thing for minotaurs or demons, these rules can help your character reflect that (and give the family something not to talk about). Finally, this chapter finishes with what most will think is its most useful section, the racial paragons. These are three-level prestige classes which grant bonuses that emulate the most stereotypical traits of a race (Stonecutting and constitution for dwarves, for example).
Chapter 2 is all about the classes. This starts with variant classes, which are your base classes, tweaked somewhat. For instance, the cloistered cleric is a variant class that drops some of the cleric abilities (it lowers the hit die, for instance) to grant such abilities as Lore and the Knowledge domain. These are actually quite interesting. This is followed by an extensive section on variant specialist wizards and then rules for spontaneous divine casters then variant rules for various class abilities, such as turning undead and the barbarian's rage. Next is the prestige class section, but in this section, they take three base classes (Bard, Paladin and Ranger) and they present them as prestige classes. This will be particularly handy for games where, for instance, a person must petition to a holy order to become a paladin. Next come Gestalt characters, which are essentially characters that have two classes at once (as opposed to multi-classing) for games where there aren't enough players to cover all the class bases (are you starting to see why no one can use all these rules at once?). Finally come the generic classes, which are a way to step away from all the class complexity and get down to four very basic choices.
Wow. Seems like a lot doesn't it. We just finished page 78.
Chapter 3 is building characters, and no, this doesn't have the old Unearthed Arcana's stat rolling system. It starts with alterative skill systems and rules for complex skill checks. Then it moves onto character traits (which are like advantages in other games). Next comes . . . you guessed it . . . character flaws. Next come spelltouched feats for those characters that have had a lot of exposure to certain spells. This is followed by rules for grouping weapons by type for the sake of weapon group proficiencies. Next comes a set of alternate rules for crafting items (magical or otherwise) during campaign down time. Finally comes background rules, for representing skills a character had before becoming an adventurer.
Is your head spinning yet? Mine is.
Next comes Chapter 4: Adventuring. This is where things really start contradicting themselves. It starts with class defense bonuses, like in Star Wars, and moves into armour Damage Reduction. Then it moves into rules for having armour convert damage instead of stopping it outright. Then it moves into an injury system that negates the use of hit points completely. But wait. Then it bring hit points back in the form of vitality and wound damage (like in Star Wars again). Next it goes back to the original hit point system, but allows for a character to have "reserve points" which essentially allow them to heal very quickly. Then it moves on to alternate rules for massive damage while throwing out a rule for dodging when it isn't your turn (a page layout nonsequitur). Next come new death or dying rules (which look a lot like the rules for dying in the vitality points section, but we're back to hit points, now, remember?) Then we move on to action points, which characters can spend during a game to help save their proverbial bacon. Next comes combat facing rules (which I've been waiting for forever) with some extremely ineffective luck rules thrown in a sidebar. But wait! Maybe you're an old GURPS player. We better throw in hex rules as well. Speaking of GURPS, who cares if this is D20. Let's take out the d20 from the game and have the player's roll 3d6s instead. Speaking of that, lets have the players roll all the dice, taking the load off the DM. And, and, and . . .
Oh. I guess that chapter ends there.
On to Chapter 5, because we've barely even touched . . . Magic. Lets start with rules that give a character a magic ratting, based on all their multiclassing, instead of a straight spellcaster level. Hey, rogues pick up stuff about magic too. Then lets introduce the concept of themed summoning lists, because it's always embarrassing the summon an amphibian on the lip of an active volcano. While we're at it, let's let characters throw money at the problem of metamagicked spells instead of increasing the spell slots (and drop in a rule about metamagic and sorcerers while we're at it). Wait! That reminds us. We have all these new possible spontaneous casters now. Lets put in metamagic rules for them and a second optional rules for sorcerers, to boot. Speaking of spontaneous casting. How about spell points? A lot of people play Rolemaster, don't they? Speaking of that, lets have characters recharge between spells, eliminating the hard cap on spells per day entirely. You know, I've stopped mentioning the side bars completely now. Still, I have momentum, so lets move on to legendary weapons, which increase in power with the character (new prestige classes in here). You know, that sounds kinda like a familiar, so lets throw in familiars that are items. Now let's shift gears and throw in rules for ritual magic (we'll call them incantations). Since we've now brought magic into the hands of even nonspellcasters, lets finish up with . . .
. . . .Chapter 6: Campaigns.
In case we hadn't, you know, done enough to shake up your game.
Let's start with new rules for contacts, but contacts need to have opinions of the characters, so we'll move onto reputation rules. Hey, didn't reputation very first come from Oriental Adventures? Yeah, let's throw in honor too. You know with honor comes the opposite, so we should have taint rules as well and if we're going to have tainted characters we'll need, you guessed it, tainted prestige classes. Wow. I'm starting to lose it here. I think we need rules for sanity! That was a little much, so we'll change tracks again, this time attacking the concept of prerequisites. How do you know if a character has the toughness feat? Let's base prerequisites on tests instead. While we're breaking free of molds, how about XP awards that aren't based on level? That sounds good so lets turn the page to . . .
. . . the afterword? Are we done already? I'm barely even started.
I don't know if you became as exhausted reading this as I did writing it, but now you should have some idea what's in store in Unearthed Arcana. Some of these rules are very good. Some of them are stupid. I doubt anyone will completely agree on which are which, and I think that was their attention. The biggest thing to keep in mind is to watch how these rules interact. For instance, if you use vitality points or the death and dying rules, character death is based on fortitude saves, so you best make sure your rules keep those from getting out of control. If you have one gestalt character you better have everyone play gestalt characters. Also, don't try to integrate rules which oppose one another too greatly, like the injury rules and vitality points, unless you want to play with real world insanity as well.
The biggest drawback of this book is you're going to waste money. No one can use more than half of these rules at a time without verging on the ridiculous, so unless you have an extremely high turnover on campaigns, it will take years to use the whole book. Meanwhile, you're paying for development, paper and colour ink that you're not using. But If that doesn't bother you, this is the book for you.
Rated by buyers 
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It is a very good book, which can give to a GM the versatility of new rules.
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I admit it. When it comes to Dungeons and Dragons I could never leave well enough alone. Aparently neither could the compilers of this cornocopia-like compendium of house rules and alternate systems for everyone's favorite RPG. For all those out there looking to spice up or revitalize your Dungeons and Dragons experience, this is your resource. Reading trough, one wonders if it is humanly possible to find time to use and apreciate all the options found within. Perhaps that is the true beauty of the thing: that the options persist in being inexahustable. With such a resource one never needs fear falling into any kind of hack-and-slash RPG rut. Game getting trite? Mix in healthy dose of Unearthed Arcana and Hey-Presto! You've got an entirely new game going! I recomend this supplement for all Dungeons and Dragons RPGers who belive that the game could be so much more.
Rated by buyers -
Ok, its time to start a new campaign and I've played everything there is to play (aside from the myriad of prestige classes that don't interest me). What am I going to play you ask yourself for hours. If you have ever had this problem, like me, probably because you've been playing D&D for far too long, this book is your savior, plain and simple.
Back in second edition there was a game called Player's Option, if you recall, and it gave you a good deal of freedom to customize your character. This book goes well beyond that. Its actually quite interesting. There are little tweaks for all sorts of races due to climate or elementally based races. Follow this up with a lot of variants for classes which are basically swapping a few things in and out in most cases, sometimes losing a trait from one class and gaining another, while some are new. In my opinion, this adds a lot more depth to the game because if you do want to play one of the 10,000 prestige classes, it gives you more than one avenue to get to them. If you do not like prestige classes, then this gives you more options than the dozen or so base classes that currently exist. I would particularly recommend this if you're fond of playing Specialist Wizards (in which case this is a MUST), Barbarians, Monks, or have ever wanted to see a quality representation of an "Anti-paladin." This may be the section of the book you most commonly use, and that would be mostly at character creation.
There are some feats, which are so-so on the usefulness scale but are just so very interesting and characterful. Then, traits to make your character more attuned to how you envision his personality, and flaws to give him special vulnerabilities (and of course there is a benefit to them as well). Its an interesting section to say the least but after that you get into some very wild and, indeed, very fun options, including a variant on weapon proficiency based on weapon groups, variants on armour systems, D&D without levels, and even D&D without hit-points. All I can say is, despite how crazy that sounds, someone obviously put a good deal of thought into it since it is at least mostly viable.
Basically, if you're bored with D&D sometimes, this book is curry powder. It would DEFINITELY spice things up again.