Fireball is almost useless - for some reason this spell damages anyone in range including you, whereas other spells don't.
Probably hammer is the most popular spell for mages to use with lightning storm a close second - hammer does less damage to less enemies but paralyzes enemies for a few combat rounds which is really useful.
The only real value I ever got from fireball was in the final fight of the game my second game. The enemy boss summoned help - an ally who cast fireball every round. LOL the ally hurt the boss almost as much as I did that game.
