I currently play a Wu Jen character (we had our second session last night!) The spells are definately a bit different than what you'd get as a wizard, but not so much that it's a huge departure. And the watchful spirit class feature (especially combined with the feats that modify watchful spirit, found in Complete Mage) is definately a lot of fun to play with - re-rolling initiative to act first means that I can really play a buffer/summoner mage reliably.
The taboos, though, are the real fun of the class. I chose some fun ones (can't sleep during a storm; can't leave a building the same way he came in), and they've really developed the character. Last session, I was stuck getting absolutely drunk so that I'd be in a different state of mind when I left the "dungeon" we were in, because I had to take the same door.
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Some other classes I've seen:
DRAGON SHAMAN - great character at low levels, as it pretty much insures no one's gonna die (even one round of fast healing will stabilize your companions, and most monsters won't knock you straight till -10 until you're around 5th level or so). The player that ran it, though, got bored around 8th level, and switched to a sorcerer.
In my play group, we have a dragon shaman ran completely differently. He's definately alright, and uses the breath weapon fairly often. I like the use of auras - he chose completely different auras than the one I GMed for, and it's nice to see the same class ran in a completely different way.
WARLOCK - the warlock I GM for got nuked in only a few sessions. He was an alright character that was fairly useful, but he didn't really stand out. He wound up getting knocked unconscious by a naga's lightning bolt. And then he fell thirty feet.
Splat.
WARMAGE - seen it a few times. It's an alright class, I guess, although I think a sorcerer is probably more useful. In fact, in our current group, we have a sorcerer/wild mage and a warmage - and the sorcerer definately comes off as being more useful. Pure damage just has so many uses, you know?
Psychic Warrior - Like it. Just saw one being used last night, and it's an alright mix. I think straight fighter is probably a better build, but we'll wait for a few sessions before I make a final decision.
SPELLTHIEF - my favourite class! even if it's got limited playability, it's just so much fun when you steal a spell. It's got "DM Determined" useability, meaning that if you have a GM that throws a lot of orcs and whatnot at you, it's not going to be useful all that often. But if you have a GM who throws a spellcaster at you every now and then, yeah, it'll be a lot of fun. In fact, last night, I counted about three times where I could've used my spellthief. Had I but known, eh?
SCOUT - kind of a cheese class, I think. We had a scout/ranger character that would move and shoot, every round. At low levels, he was a damage machine. I guess this wouldn't be the case at higher levels (since he'd be giving up a full attack in exchange for skirmish damage), but at low levels? Painful.