OK, I have it now also.
My overall impression is that the system is innovative & interesting. I'll have to do a lot more reading through to get a sense of game balance of the new classes vs. core classes, but I do have some initial impressions.
Incarnum Classes vs. Arcane & Divine Classes
IMO, each of the classes (even the 1/2 BAB incarnate) is probably better considered a 'hybrid' class than a magic-using class. There are meldshapes that boost skills, that boost resists, that boost ability checks, that boost weapon damage, that provide natural weapon attacks... none that really provide an alternative to Cure <Whatever> Wounds & not so many that provide an alternative to Fireball IMO. I suspect (very preliminary impression here) that meldshapers -- even Incarnates -- are going to work best in most cases when using their soulmelds to supplement regular weapon attacks.
Concerning the healing capabilities, one thing that struck me is that there seem to be a lot of comparisons being made between the incarnate class & cleric. I think this is rather misleading in one crucial aspect: healing. Meldshapers simply do NOT make good healers & are NOT going to be able to replace even a bard as a healer in most games (low magic games used to dealing with non-magical healing would perhaps be an exception, but I've no experience with that sort of game to do more than speculate). One meldshape can be used to boost regular divine healing, & one can be used to heal a very small amount at the cost of damaging the meldshaper, but none can really replace standard magical healing.
I also simply do not see the meldshapes given as having the damaging capabilities of regular magic, at least at higher levels -- even though (like the warlock) they can be used again & again & again in almost all cases. For example, using the totemist's Winter Mask soulmeld with maximum invested essentia for the level (short of class abilities or feats that would override the normal level max) would let you do 10d6 cold damage in a 15' cone at 18th level. A wizard of the same level could use meteor swarm to do [2d6 bludgeoning (if a ranged touch attack succeeds) & 4d6 heat (no save if the ranged touch hits, save if it misses)] x 4 -- & all of it at LONG range. That's a ton more damage, & the greater range means at least situationally a lower likelihood of the party taking damage during that round vs. the totemist's party (or at least the totemist) being within move range of most anything that would want to focus an attack on them after they used the Winter Mask effect. Sure the totemist can keep pumping out that cone as long as they're alive & the soulmeld isn't dispelled, but the longer an enemy survives combat the lower the chance that all of the party will live.
For those (like myself) hoping for something to provide replacements to Vancian magic for the core archetypes of healer & nuker, then, this doesn't seem to be it. Instead, it is IMO a book of interesting alternatives for gamers wanting to play a non-traditional character.
Incarnum in the Gameworld
For the reasons stated above I don't think that incarnum use would balance out against the power that arcane & divine magics can offer to warfare, a consideration important to believability in worldcrafting. The classes do look interesting & fun, & obviously having them around as another option mixed in with everything else in a society would be fine. I'm also very much of the mind that a world built around incarnum use as the only form of magic (or incarnum & psionics) would work.
However, I don't think sticking incarnum users into a high-magic world as the sole form of 'magic' for a specific area that has contact with regular D&D magic would be plausible; IMO the meldshaper societies would simply fall before armies employing wizards (& sorcerors, & warmages) & clerics (& favored souls, & druids, &, heck, even bards). For DMs wishing to place incarnum use into an area seperate from 'core magic' areas, it would probably be best to provide reasons why the societies have managed to coexist without the incarnum-using societies being conquered; perhaps their lands have only recently been discovered, travel between the regions is exceedingly difficult (making invasion impractical), or they have access to other forms of power besides incarnum which permits them to survive in the presence of magic-wielding societies.
In a low-magic setting, having incarnum as the primary 'magic' for an area would probably work; the simple rarity of arcane & divine magic & their resultingly limited effects on warfare would eliminate any major societal imbalance in this arena. This type of game could even have some interesting situations with incarnum if it is less uncommon than regular magic, perhaps an invading army of meldshapers from one area is overpowering the mundane armies & leading the low-magic society under attack to reconsider attitudes that might be behind the rarity of magic-users in order to develop effective (& ultimately overpowering) tactical weapons against the incarnum-users. I also think that a game where incarnum is the 'accepted' form of power manipulation & magic is considered taboo could be fun, & the (probable) rarity of standard magic in such a setting would again serve to counter the power differential. One of the PrCs given would actually be ideal for that type of game, or one with limited contact between incarnum-using & magic-using societies.
