Just got the book, so I'll add my 1st impressions.
The races were pointless. Not just because new, non-iconic fantasy races are hard to pull off in general, but because these races came off as a half-arsed effort. The "Human+Incarnum" race was beyound pointless. There's already a feat to gain an incarnum point, right? Only the Duskling and the Rikish had any potential, but they lost it by not going far enough. A Fey race? Excellent. Everyone's been waiting for more fey...so don't just give them one incarnum fueled ability, a bit of bland fluff, and call it a day. Rikish were similar. The image of a Rikish monk using arm spines is pretty cool...and it's about as far down the cool path as the race manages. Humans with arm spines...sigh. New races shouldn't be based around a single ability...it comes off as unatural specialization. Little bits like kobold's bonus to trapmaking really help round out a race.
I agree that the background Incarnum fluff doesn't interest much. Manipulating the stuff of souls is the most subtle form of magic? Works much better as a shaman system, which is part of the reason the Totemist comes out as the most interesting class. The mechanics and concept come together well for it.
The Soulborn looks solid mechanically, but insanely boring. Yes, I realize that the Paladin and Ranger have a similar spell progression, but they also have a host of other abilities, and the advantage of being archtypes.
The Incarnate is the class I've been looking at the most, if only because it's apparent weakness makes it sort of an instant challenge to see if you can build one that doesn't suck. I don't like the 4 alignment rule, personally, and I'd probably houserule it, but other than that, it seems interesting, aside from some built in mechanical issues.
I don't like the 1/2 Bab, I just don't. It makes no sense, and only the Lawfuls have a good way to get around it. I don't want my champion of good to spend his whole career spitting acid, just because his host of self-enhancing powers include every tool he needs to be competant in melee, except the ability to hit anything...lame.
Just as a mechanical question, I noticed Merric routinely has his sample Incarnates adding about 7 essentia to abilities at 20th level. I thought the cap was 4? I'm sure I'm wrong, but I'd like to understand the rules on that point.