No, not the spellthief! That class has GOT to be my all-time favourite D&D class... while it is a bit weak in comparison to other classes, stealing spells has got to be the most fun ability EVER.
The big problem is how it fits into the campaign. Our GM made sure to include a lot of traps/magic users in the early game, and my big problem as the spellthief was killing the mage I was attacking before I was able to steal more than one of his spells. But it was still a lot of fun to make your sneak attack, get a spell like Color Spray, and trying to figure out what to do with it. Lots of fun.
As the game progressed, though, the GM started focusing on using "big bruiser" creatures, and my spellthief fell by the wayside. But that's more a GM problem than a class problem - if you put a spellthief in a game with at least a few spellcasters or monsters with spell-like abilities/energy resistances (and who DOESN'T put at least afew of those in most adventures?) the spellthief is a lot of fun. It's also a lot of fun if you're the party's main rogue. To be fair, though, I think if you gave the spellthief a 6 level spell progression, it'd do a lot to make the class more attractive.
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I've ran a game with a samurai, although he used maces instead of katanas (nice little house-ruling, there), and he wasn't oriental in flavour. He was a hobgoblin mercenary, and the character was pretty well thought out. THe big problem was the player wasn't willing to make use of the intimidate ability, and when he did, somethine inevitably got in the way. He wound up being the weakest character in the group (human fighter, human warmage, and a half-elven scout), and most of us agreed - the Samurai is underpowered. We also agreed that hobgoblins don't deserve that +1 LA.
But, I voted for the Soulborn, for one simple reason: I really, REALLY hate the idea of Incarnum, and the name sucks. I haven't had much experience with the class, but from what I've seen - YUCK.