In WotC's first major rules expansion, they have produced a product that adds new options to the existing rules and still manages to add flavor to the game. This book gives you more prestige classes, new feats and skills, new weapons & magic items, and in perhaps the most useful section- examples of how the combat rules can be used to maximum advantage.
The new prestige classes serve two purposes: they instantly give DMs and players more options for PC and NPC play, and they show DMs more examples of how prestige classes can be built. Most of the new classes in this volume are the standard cultural variants (samurai, ninja, cavalier, archer) with a few fantasy-themed exotics (ghostwalker and perhaps the silliest of the lot, the drunken master-a character whose attributes increase with the consumption of alcohol) thrown in for flavor. WotC goes the extra yard and describes some of these prestige classes as organizations with goals and initiations so they are ready to insert into a existing campaign. The Knights of the Watch (a chivalrous knightly order) and the Ravagers (a group bent on violence and destruction) are typical fare, but like everything else they are easily customized.
The new feats and skills broaden the fighter class to the point that fighters can progress along entirely different paths and evolve into vastly different fighting machines. The core rules made this possible, but new feats like spring attack and "5 foot blindsight" can really lend weight to a player's wish to be different. Sure - you can SAY your fighter seems to have eyes in the back of his head, but if the character takes the feat of the same name, you have rules to make your fighter's persona have a real game effect. This is undoubtedly what the WotC folks were shooting for, and more power to them.
The new weapons and magic items provide tools for the diverse characters that PCs can now create. From Fukimi-bari (small needles that ninjas spit at people) to the Mercurial Greatsword (an instant favorite of the power-gamer) - the weapon selection has broadened considerably. Most of the weapons seem to have avoided the common pitfall of simply being differently shaped versions of the same stats. WotC has also thought to head-off this kind of thing by providing equivalents for new weapon types (e.g. "A Katana is equivalent to weapon X" for stats). This gives a DM much-needed guidelines to handle the player whose character simply has to have an Atlatl.
In the section titled "The Game Within the Game" WotC takes the nebulous collection of combat feats and skills and gives a player examples on how to use them effectively. As any player in their first 3rd Edition mass combat knows, the new combat rules are much more complicated than they first appear. Sure they're easy to use, but when a lot of feats and skills collide in battle, new combinations are bound to come up. WotC provides some very basic advice about attacks of opportunity and movement, and follows it up with two wonderful examples of feat-oriented combat, complete with explanation and character stats. In my experience, these kind of examples are the best way to teach people how to game properly (WotC should have put TEN of them in this book, along with one a week on their website - there's really no substitute for seeing the rules in action). They also take the time to impress on you how truly fearsome a creature with reach can be in combat, (DMs, try this: Give all the trolls in your campaign the new "Large and in Charge" feat and watch all your players suddenly develop an interest in polearms....*grin*)
The worst thing I can say about this book is that it costs as much as the core rulebooks. One look at the contents convinced me it was worth having, but its clear that WotC is bound and determined to make some money off the expansions. But so far, WotC has managed to avoid TSR's recurring problem of superceding the core rules in a supplement. By adding rules within the main framework they have created a really useful addition to the game...and they've made combat in D&D a whole lot more interesting.
So...watch your back.