Gort said:
I'm really interested in the spellthief as a concept (I like rogue-mage types

). Since you're a fan, what's it like in actual play?
The spellthief is very GM-dependant, in that if you don't have a GM who is going to throw in enemies for you to steal spells from, you're screwed.
When we started playing, I had a GM who was more than happy to throw in a mage or something for me to steal spells from. And, trust me, that's a lot of fun. When you're in the middle of a big fight and you've got a color spray spell that you've never used before, you're going to be thinking in ways you're probably not used to. It's a class that NEEDS a creative player, and I like that.
Unfortunately, that same GM started using more and more "bruiser" types, and wound up not even throwing in the occasional trap for me to disarm. The last useful thing my spellthief did was rescue a few children from a fire, and that wasn't really class-related. My spells weren't entirely useful, either (I played till 6th level) - casting Shield before a fight with us was pretty much spending an action to get an AC only slightly lower than everyone else in the group.
The big downside of the spellthief that will ALWAYS be a problem is that if you're going against mages (the big enemy for the class), odds are good that you'll kill your target in the same attack that you steal his spell, which is fine mechanically, but I think ruins the flavour of the class. Also, in any particular encounter, you're only going to make one steal, and it'll take you at least two rounds to do so. Since a combat is seldom more than three rounds long, your actions look like this:
ROund 1: Get into position to steal a spell.
Round 2: Steal a spell.
Round 3: Cast spell.
Round 4: Loot the bodies.
Whereas, in that same round, the sorcerer is:
Round 1: Cast a spell.
Round 2: Cast a spell.
Round 3: Cast a spell.
Round 4: Loot the bodies.
So, yeah, the spellthief isn't entirely useful. But it *IS* fun.