I must confess I have had a couple of periods (about 2-3 months each) where I didn't have a job, and all I would do is play WoW and get high.
Towards the end of these periods, I was a wreck. I would look extremely pasty and unhealthy, had little or no social life, and ended up rather unhappy. All the hallmarks of addiction. Lo and behold, if the servers went down (which they did frequesntly at launch) I was not a happy bunny.
For the other 18 months or so that WoW has been around I have either been taking a break or playing about 7-8 hours a week. Now I normally play a couple hours on a couple of weeknights and most of Sunday.
Obviously it is far more important to have a real life. Work hard, Have a social life, get outside, meet people etc..
A wise man once said "time wasted having fun is not time wasted at all". I have to disagree. So many opportunities can be wasted by staying indoors playing WoW all day. When you are on your death bed, you won't looking back on your life and say "Well I got that uber epic weapon so I had a good life."
All in all, WoW is a lot of fun and can be addictive and habit-forming as hell. No one will realise this until they experience the bad effects themselves, and thankfully most people wouldn't let themself get this far anyway.
Just remember it's a game, all things are good in moderation.