Ex-WoW addict speaking. (I wouldn't call it extreme, but I was playing quite a bit)
DON'T START PLAYING
I beat my addiction a while ago and If I wanted to resume playing casually I could but trust me, if you pick up WoW now its completely fresh to you will get addicted if you enjoy the game. The thing about MMORPG's they compel you to keep playing. There is no 'GAME OVER' there's always more to do (AKA Achievements, basically a copy of xbox 360's system), it appeals to peoples competitive nature because you always want to be more progressed than someone.
Luckily for me this was in yr 8-10 so nothing too detrimental, but it does fuck with your lifestyle just really if you want to play its a good game but wait till your HSC is over.
Nowadays if I'm bored i'll just have a quick game of dota or play some fifa 10.
PS Really don't play, even if you think you have an iron-will don't.