in MMORPGs mages historically have been difficult to level, as they are innately weak against melee enemies.
but, as with any class, if you know what you're doing mages are deadly no matter what level.
i'm a rogue myself. you deal a lot of damage, can absorb a decent amount, and have some really fun skills and talents. using stealth to your advantage and also the use of poisons makes it very powerful and a lot of fun.
beware shamans and priests though, they are a rogue's worst enemy. it gets easier once you have poisons.