1) Warlock: He is the master of early game aggression. (yes, even better than the BM) I feel like early fights require a lot more tactical skill than lategame fights. When you have very few creatures, enchants and conjurations, every move and every action becomes far more important, and the order in which you activate creatures becomes amazingly important.
2) Wizard: The most versatile of the Mages, the Wizard is whatever you make of him. You can play aggressively, defensively, creature-based, attack-based, etc. Any way you want it, that's the way you need it... there's a reason he seems to be the most popular Mage.
On the downside, the old Wiz feels like the most fragile of the Mages. Not because 32 health is that low (it isn't, and Voltaric Shield is great) but because he pays 2x spellbook cost for every healing ability in the game. At least in my experience, even the warlock heals for more with Vampiric and Deathlink.
3) Priestess: I like her because she can choose to play very aggressively or very defensively, and her Daze/Stun abilities can really shut down other players. Healing is very strong, and as long as the Priestess can keep a core army of level 3-4 units alive she is likely to win. The downside is that she is vulnerable to Finite Life and Nullifies, and since there is no Holy familiar she can find herself at an action disadvantage against multiple curses.
4) Beastmaster: In my opinion, the BM is like a weaker version of the Warlock. If you just read his character-card abilities, they seem stronger than the Warlock's. Yet after dozens of games I just feel outright stronger as a Warlock. I suspect that Ring of Fire and Fire Immunity has a lot to do with it. The Warlock is the only Mage that can freely blast away with zone attacks while also swarming with creatures.
5) Warlord: The Warlord is incredibly crappy in the early game but becomes exponentially more powerful as you let him build up an army. The problem is, there are so many early game aggression strats out there that Warlord players are often forced into playing the Round 2 Sniper Tower just to defend himself. The sniper tower is a strong play but it is a bit of a one-trick pony.
6) Forcemaster: I have no idea how to play a Forcemaster. With the exception of the very first Forcemaster vs Warlord game I ever played, I really haven't seen FM win any games. I'm not sure why she feels so weak, because her ability card seems extremely strong and forcefield seems extremely strong. I think it has to do with the combination of paying 3x cost for most creatures, and paying 2x cost for non-mind incantations and attack spells. If you only play in-school, you simply don't have any heavy-hitters other than Galvitar+Scimitar melee, which is easily avoided by Guards, teleports, tanglevines etc. As soon as you start taking many out-of-school spells, your spellbook gets so thin that you run out of options early in the game.