I was addicted to Dark Age of Camelot for ages, until Trials of Atlantis came out. At that point my interest started to wane, as it took insane amounts of time and people to succeed, and the expansion added to an imbalance that was already there on my server. Midgard was very powerful at that point, until NF came out, when Albion pretty much tore up every server, and my poor Hibbies didn't stand a chance at all
All in all I lost countless hours of sleep playing that game, and I don't regret the time i spent playing it. Hopefully Guild Wars will offer that same feeling when it comes out.