I am a Fighter/Mage.
This is one of the most powerful combos in the game. Especially for BG2.
For most of BG1 you are going to be 1 level behind a single class character. If you have the ASF mod (you are allowed to wear armor, but incur ASF like in PnP) then wear armor until level 5 or 7 when your mage powers start to shine. Otherwise stick to a reach weapon and let someone else tank...
in BG2 you really shine, most high level buffs a mage can cast are SELF ONLY. as such a fighter/mage is:
1. The greatest tank in the game
2. The greatest melee damage dealer in the game.
3. The most effective class for wielding staff of the magi, one of the only two weapons which dispels on strike (staff of magi can only be wielded by mages and their multiclass) which dispels each enemy it strikes. The only other weapon that does that is a holy avenger called carsomyr which only a paladin can wield. Of course an epic rogue can wield both with their "wield any item" ability.
There are a few other awesome weapons, you should look them up OR save edit your character's proficiency choices as you come across them.
4. The ability to strip an enemy mage of his defenses with a spell before finishing him off. (true sight, breach, dispel, stoneskin, immunity to magic weapons, and immunity to abjuration are some of the most awesome things ever) is extremely useful. You are eligible for 1v1 mage battles which you are going to absolutely dominate (breach/dispel followed by melee them to death).
Seeing as you are now in BG2, get a robe of archmage or better yet, robe of vecna!
I seem to be drawn to evil characters
They are the most funny and interesting of the bunch. In BG1 the companions are all completely silent anyways. In BG2 they have 1 or 2 banters unless they are one of the 3 romances, which are just plain awful...
That being said, if you like playing good expect all the evil characters to LEAVE you once your reputation hits 19 or 20 (out of 20).
So just toss them now and go get the good guys already.