I get the damage bonus for the barbarian (never really noticed it helping out though but sometimes things are too hectic to look), the awesome killing power of a mage, and a little healing ability (3 times) in case things get rough.
I'm wearing a coat of plate and I don't notice any difference in my magic ability. I'm wondering if that really is working like they said it should.
There is no doubt clerics are powerful (as my necromancer king can testify!), but for levelling power you can't beat the mage. I am not sure how much damage my fireball does, but the AOE spells gets me a lot of XP and my highest level characters are always mages of some sort, simply because magic enables you to kill more foes at a time.
Although at the higher levels and in bigger battles, I've found that my power is better used for things like Chaos, Slow and Haste, as they make a huge difference! The support-style mage is nothing to be scoffed at, however tempting it is to rain fire on your enemies. As Morrowind Mod Man said, Haste works wonders.