I'm currently playing a soulknife, when my group is able to get together. At 6th level, he's half the party's front line, alongside the party's werebear PC. I really enjoy playing him. His biggest weakness is a low AC (by choice--so armor check penalties don't interfere with his tumbling or jumping checks). But with combination of lucky rolls, a high Con, and the Psionic Body feat (and subsequent psionic feats) he has 84 HP.
His favorite tactic is to keep his mindblade charged with his Psychic Strike ability, and then boost it with Psionic Shot. Before engaging in melee, he uses the Throw Mindblade ability to start with, then manifests the mindblade again, uses a move action to enable the Psychic Strike, and wade into the fray.
I really enjoy playing him. Sure, like Psion pointed out, it's a rather narrowly focused class, but he's been a blast to play so far.