For me an MMO is only fun if your at the top or close to it, which means getting great gear and getting into a great guild and then raiding 6-7 nights a week. Unfortunately I did that for a long time and eventually quit and sold my account on ebay, and promised myself I'd never touch an MMO again. 150 days played down the drain, and that was in WoW well before TBC came out.
It's not just the time that it takes from you, it also becomes the centre of your thoughts and somewhat of an obsession and an addiction, at least for me I found. Even Diablo 2 which I find addictive as crack has nothing on WoW (and likely other mmos).