Doing well Frost is as strong as ever imho for leveling, the new spell effects look clean and nice. Sure we lost our elemental makeup of MOP, but then other classes have a lot more. Frost is so far strong and looks good on paper. Fire has unlimited LB cleave back and Arcane is okayish. Going by the playstyles, i am definitely enjoying the class over my previous main of 9 years (warlock)
I am thinking mages will be fine in the new xpac. I am ignoring nos, because that's currently a moving target, but we should be tuned fine.