I've played two MMOGs. Travian had me hooked where I got up in the middle of the night to check in and play. Took me a bit, but I ended up leaving it behind.
Started playing Heroes of War and Money, an online version of Heroes of Might & Magic, and quickly realized it was heading into the same place. Since you couldn't delete your account there, I had to get creative... changing my account email to one I don't use and changing the password to a randomly generated string that I never saw. There's no way to get back to that account... and that's a good thing
Addictive? Yeah. Heroin? Probably... except I think heroin is probably less addictive.