Now that I have the book and have actually sat down and given the book a good read-over, not to mention taking it for a test drive by making some characters, here's my $.02. For what it's worth, this is for someone who's initial reaction was "this is interesting, if overpowered." I specifically disallowed these classes from the Shackled City game I'm running, for example. Everything in this post is just my opinion, so, in the words of Conan O'Brien: be cool, my babies!
Now that I have some experience with it, I'd say the book fits with my style of play and will likely help me make D&D more the kind of game I want. What does that mean? I am not overly excited by the number of magic items that make their way into a typical powered D&D campaign at high level, and much prefer characters to be able to rely on their class abilities as opposed to their items.
For spell casting characters, this is not too much of a problem: a high level spell caster has many ways to be perfectly viable with minimal magical items. For non casters, however, this gets to be a problem, as a high level fighter needs to either be supremely cheesed out with feats and prestige classes, have a huge amount of magic, or both in order to be competative. I think this is a somewhat contraversial opinion on ENWorld, but I've never really understood why: the games designers have talked about the fact that there's still a lot of "suck at low levels, rule at high levels" in terms of spell casters.
To my mind, the Tome of Battle does a good job of bringing fighting characters up to the level of the spell casters at the higher levels in a relatively simple and straightforward manner.
That last part is the key. One of my players routinely posts to and reads from the optimization boards on the WotC boards, so I know fighter-type characters can be effective at high levels, believe me. When you see people talking about the ability for a high level character in Tome of Battle to do 100 damage with an attack, you see people breaking into two camps: those who say "OMG! That's crazy!" ... and those who say "meh, amateurs!"
So for me, if I were running a campaign where I was keeping a lid on the number of magic items in the game yet I still wanted to run it at the power level of a standard D&D game, Tome of Battle would be just the thing.
In a traditional campaign with normal items, I think these characters might be more prone to abuse. It is really hard to say, though, because the characters I've written up were really not that earth-shattering. Part of my realization of how this works was actually creating and leveling up some sample characters.
The warblade is really limited in what maneuvers he can select due to the prerequisites system, which is something you see in leveling up characters but you don't necessarily see when you try and quickly create something at an artificially high level. If you want to learn one particular maneuver, it's easy to do a build, but this is an extremely limited character, and also one that will not level up terribly well.
For this reason alone, I'd strongly suggest not jumping to any conclusions about the classes until you actually have the book and use it to make up some characters (unless, of course, you hate the very idea of fighting styles and maneuvers for your game).
Maneuvers are a genuinely new mechanic for D&D. You can't talk about them as being balanced with feats or spells, because they're really something new: they're half way between a feat and a spell, and the method used to limit how often they can be used is something that will take a while to get used to. A maneuver is more powerful than a feat, but it's also limited in how often it can be used (in our warblade example, every other round as a best-case scenario) so it's generally less powerful than a spell. Almost all of the maneuvers affect a single target and can't have the effects of other feats such as meta magic applied to them. There's no "quicken maneuver" ability out there (at least not yet...) They also are one round effects that typically use a standard action to use (for strikes anyway). That is a huge limitation, because you can't use them with any feat that talks about the attack action, nor do they stack with effects like haste. With all that put together, I'd have to say that a maneuver is the new guy in town, and we will have to see how it balances out over time.
So overall:
Personally, I think the mechanic for balancing powers by the encounter is the right direction to head D&D towards. Tome of Battle is just the beginning of any movement in that direction, and it's something I'd like to see more of.
The only thing I'm going to be super critical of is the book's organization. I'll join with many others who say that it is just way too hard to actually set up a character under the maneuver system. A master table including the all-important pre-reqs would be a godsend.
So there you go.
--Steve