I definatley think that online games are addictive, and the more time you put into them, the more addictive they get. HOWEVER alcohol and all other drugs are addictive but it is up to the user to control their actions and not allow themselves to become addicted to a point where the 'habit' becomes a problem.
I really enjoy playing WoW, and if I have a day off work and my GF is not at home I will sometimes play all day, this makes me want to play more.
But the average time I devote is about 8 hours a week, on two evenings with a bunch of good freinds, and that keeps me happy.
TBH though I found Baldurs gate (a non online RPG) more addictive than any MMORPG i've played since. (apart from possibly EvE Online)
Hope that helps.