I've played through the game 3 times now, once with each class. I always used Morrigan, but never her shapeshift ability, because a mage with no spells and more health is just a really shitty tank.
I just used frost/fire/telekinetic weapons and the arcane damage moves, like fireball and Blizzard.
On that note, did anyone ever get the hang of the whole "spell combo" thing? Like casting grease and then fireball?