I was really struggling with my mage at the beginning, but after a restart and different approach I find them very fun to play.
The key to making them usable at least in early game, is to spread out their points. Get a point in hydro, aero, pyro, summoning, maybe some damage reduction as the new AI knows who the weak one in your party is. Also, gear will come with random +1 to different skill trees, i.e. geomancer, etc... So use that to augment your characters so you can do different types of damage.
I completely agree with this. My team is made up of 2 mages, 1 warrior/necro/morph, and 1 ranger.
I've tried many different skill distributions with my mages but what I find to be most fun and effective is the Jack-of-All-Trades approach. At level 14 it looks like this:
Warfare: 1 pt (for executioner)
Huntsman: 2 pts (for tactical retreat)
Polymorph: 2 pts (for Chameleon & Wings)
Hydro: 2 pts (healing and magic armor healing)
Aero: 2 pts (teleportation, nether swap)
Earth: 3 pts (AoE slows, poisons, oil fields, armor recovery)
Pyro: Remaining points (Damage)
Wand & Shield
Even split between Int and Memory
With this setup my mages are masters of the battlefield and are prepared for any situation. They have high survivability, high mobility, high aoe damage, great control and are a blast to play!