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.

Dungeons & Dragons 3rd Edition Classes
Player's Handbook: Barbarian - Bard - Cleric - Druid - Fighter - Monk
Paladin - Ranger - Rogue - Sorcerer - Wizard
Player's Handbook II: Beguiler - Dragon Shaman - Duskblade - Knight
Complete Adventurer: Exemplar - Ninja - Scout - Spellthief
Complete Arcane: Warlock - Warmage - Wu jen
Complete Divine: Favored Soul - Shugenja - Spirit Shaman
Complete Psionic: Ardent - Divine Mind - Erudite - Lurk
Complete Warrior: Hexblade - Samurai - Swashbuckler
Dragon Compendium: Battle Dancer - Death Master - Jester
Mounteback - Savant - Sha'ir - Urban Druid
Dragon Magazine: Sha'ir - Deathwalker - Fleshcrafter - Soul Reaper
Dragon Magic: Dragonfire Adept
Dungeonscape: Factotum
Eberron Campaign Setting: Artificer
Heroes of Horror: Archivist - Dread Necromancer
Magic of Incarnum: Incarnate - Soulborn - Totemist
Miniatures Handbook: Favored Soul - Healer - Marshal - Warmage
Oriental Adventures: Samurai - Shaman - Shugenja - Sohei - Wu jen
Psionics Handbook: Psion - Psychic Warrior - Soulknife - Wilder
Tome of Battle: Crusader - Swordsage - Warblade
Tome of Magic: Binder - Shadowcaster - Truenamer
NPC Classes: Adept - Aristocrat - Commoner - Expert - Magewright - Warrior
Class-related things: Favored Class - Gestalt character - Multiclassing
Prestige classes - Variant Classes - Epic Levels