Magus was an interesting villain and when I got him to my group... whoah! It was interesting to know more about Magus's history in Zeal Kingdom and such and it deepened his character a lot, IMHO. He just wasn't an evil wizard trying to destroy the whole world but had other interests. :D
Anyway, I didn't use him much as a party member though because I considered other characters better fighters. I used Magus in some minor parts of the game and I also used him to fight Lavos at the end of the game. I tried different party combinations to beat it so Magus was included too although I didn't pay so much attention to him. Magus just isn't much of a team player like some others have said in this thread. :p