Soulborns are basically paladins who use incarnum in that they must be of one of the extreme alignments, they smite, get defenses based on their alignment and suck hard. Like the paladin, they get their main shtick at level 4 and they're horrible at it. 5 soulmelds at level 20, 3 of which can be bound and 10 essentia to share between them? The totemist can do the same thing (except with 3 less essentia and one more soulmeld) at level 10. Same for the incarnate, except without the difference in essentia. Yeah, some of their unique soulmelds can be useful, but you can just nab them with a feat (meldshaper level at half the character level, though). Of course, since the soulborn has all of its meldshaping capabilities at half its soulborn level, you'll be using them at the same power level, while not having to be a soulborn. Unless you use a good fix, don't play one. Soulborn are the bottom of Tier 5, only avoiding the fall to 6 by having effects that are actually useful (if they can be obtained by a feat) and a strong candidate for the second worst player class in the system (the worst is indisputably the Complete Warrior version of Samurai).