I primarily played the mage class, but didn't use a lot of magic (mostly just for big fights). After finishing most of the game I was a level 6 thief, level 8 fighter and level 17 mage. At that time I couldn't sleep for a couple nights in a row, so got up and went through UnderSigil (I was too tired to want to head to the final area). Anyway, after killing time going through UnderSigil repeatedly I became annoyed the boss monster wouldn't drop anything useful, so went through a few more times, then gave up and headed to the final portal as a level 28 mage. Towards the end of the final area, before using an object that gives an experience bonus, I was level 31. Just before the final encounter I was level 38.
Playing again I would stick to being a fighter (though of course doing the quests to become a mage and thief). I'd leave intelligence and charisma relatively low (IIRC 12 or 13 is good enough for the majority of stat checks), and boost strength, wisdom, constitution and dexterity (in about that order).
