I rolled my "f2p" mage because I already had a priest, and wanted my f2p-restricted worgen to be as trollerific as possible. I'd played one a bit in the past, and I had a lot of work to do improving as a DPS (and in general) in PvP. The mobility and CC are great, and they offer amazing utility in a strong group. Balancing various forms of CC while being super squishy was a great learning experience, poor positioning and awareness almost inevitably puts a Release Spirit button on your screen.
With 5.0/MoP on the horizon, I've finally started looking through the new talent calculators a bit, and I'm pretty unsure about mages at 20 going forward. Cone of cold is moved to 28, Improved CS to 70, Frostbolt is limited to Frost spec. As arcane, the only way to slow someone will be a 2.75 second Frostfire Bolt cast.
Arcane Charge - Spell - World of Warcraft could be a nice boost to damage, but the cleave worres me slightly in terms of breaking CC.
Frost seems relatively unchanged, outside of Cone of Cold.
Ice Floes - Spell - World of Warcraft from the T1 talents may be enough of a mobility buff to make it the ideal spec.
Off-topic: Ice Lance at 22 and Fingers of Frost at 24 are scary.