I've only played BG1 but I've been having loads of fun with my Mage/Thief. I've got magic when I need it and I've got ever important and fun thief skills. I wanted to go plain mage but I died to fast so I combined a thief with it, giving more HP and better weapons to use without having to lose heavy armor wearing capabilites (Like in a Fighter/Mage or Fighter/Cleric). Also, with the huge number of good Long Swords in the game, I can also put those to use if I ever get more than my fighters need.
-Jafin
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I am Jafin, arch-mage of the HADB clan!