You can get pretty addicted to them.
I played WOW for a while, not on the official servers but on free servers (private servers).
Thing is, I only ever got to like 25 or whatever and then I got bored and stopped playing & deleted it. Suddenly I got this craving to explore that world again and walk into those large gates of stormwind city.
Then the same happened, got bored & deleted.
Same happened again, I was dying to kill some more of those evil dark creatures.
Then I got bored again.
Same happened again so I re-downloaded & re-installed for the last time.
Then got bored and decided to give up and I've not played it since (this is about 5 months ago).
So woohooo!!! I've fought the powerful addictiveness of mmo's!
Now i want to get back into games like Diablo 2