I'm only sharing my story as a sort of fresh example amidst the many not-so-happy endings (and fully justified warnings) that are posted here. My experience with MMOs has been nothing short of positive, it's had its ups and downs, but I regret none of it whatsoever.
Arguably I first started 'multiplayer' gaming with Diablo II back when I was in elementary school, when I had to sneak around my parents strict 'one-hour per day' internet allowance to myself and my siblings (all of whom shared the same computer). Needless to say it was very hard, which probably (definitely) owed to how I pretty much exploded into the world of online gaming the next time I was exposed to it.
I'd graduated, and was going to college away from home when my boyfriend-at-the-time burnt me a copy of the Ragnarok Online client for a private server (which will remain nameless). Long story short, I went from being a player, to a GM, a PR representative and then the head administrator for a modest population of 500 players. It was in this position that I experienced and embraced my first real taste of MMO subculture. It took a lot...no...all of my time, but I loved every moment--being an authority figure, a player, a friend, a lover and essentially part of the creation and life of the server. It was an experience that shaped me in a way that I would trade for no other.
'IRL' circumstances forced me to move back home and transfer schools. My experience on the server inspired me to finally declare a major in Computer Science. The new distance strained and eventually broke my relationship with the man who had first introduced me to RO, and after a two-year stint working for the server I passed the torch to a most capable sub-admin who did me proud in maintaining the server for a year afterwards.
However before I left I happened upon a pair of gamers that'd just joined the server, one of whom nonchalantly mentioned (several thousand times) ingame that his friend noticed I lived in the town next to his (thank you, Myspace). Eventually he coaxed/harassed me into meeting his friend, whom I've been dating for almost two years now. After leaving Ragnarok, we spent about a year with Dark Age of Camelot, and eventually moved to WoW. Gaming is a core part of our relationship, and (yes, I've seen a screening of Second Skin) upon seeing one of the first scenes where a gamer's girlfriend logs her boyfriends character into WoW so he can make a raid spot I immediately thought "That's us!" No seriously, we don't live together yet (graduating soon!) but I physically bring my computer to his place when I'm able to visit for extended periods of time. We go out for dinner, a movie, and come home to raid or PvP till the wee hours. His buddy that practically hooked us up? Playing WoW with us, as are several old internet friends that the two of us have gathered over the years.
Whats the catch? There is none. I'm taking 20 hours of classes per week and maintaining grades that I'm pretty proud of. I additionally work 25+ hours a week at a job I've had for 3-ish years. I'm social, I spend time with my family, and I play WoW every chance I get--averaging 3-4 hours on school nights. Is there temptation to pull all-nighters grinding BGs for honor or raid dungeons for loot? Sure. I remember last summer on the weekends I would sometimes spend about 10 hours running around the frontiers on DAoC because I had the day off, but I was also socializing with people that to this day I still talk to via AIM despite the fact that I havent played with them for a year.
Basically all I'm trying to say is that MMOs can be a fantastic experience. My story isn't over yet, but I think that even if I do experience a nosedive in my offline life due to my gaming 'habit,' that (post-recovery) I still won't regret how much time I've spent, and won't easily forget the good times I've had.