I play Guild Wars more than enough.
(I was big on PvP for awhile and still enjoy it, but I just don't have the time at the moment to pursue a top 100 PvP guild. So I"ve been doing a large amount of PvE the past few months.)
GW is not quite MMO, and that lack of true MMO is one of the few gripes I have about the game. No monthly fee is a great plus. They're gearing up to release Guild Wars 2 I think, so I'll look closely at that.
Before GW i played Star Wars Galaxies. I started SWG at launch and had a blast for the first 9 months or so. It was by far the most fun I've ever had gaming. Then Sony Online Entertainment screwed it all up. SOE kept adding more crap and really didn't fix any of the core problems with the game. Then it took me 9 months to cancel my subscriptions. There fore I will never, for the rest of my life give a dime to SOE.
Now I look t the future... I'm keeping an eye on Star Trek Online as it develops.
Non MMO:
I love SciFi, and really really dislike the D&D genere. Guild Wars was soooo good for PvP though that it was an exception.
I played Diablo 2 LOD around patch .09 but couldn't stand how poorly Blizzard managed the game, and how rampant hacks, and exploits were. I still play it now and again with my younger brother but not for personal enjoyment.
I loved Star Craft but again was realy dismayed at how poor of a job Blizzard did at fixing exploits.
So I stopped playing some time ago. These negative experiences with Blizz made me shun WoW from the get-go. Though I might be able to bring myself to play Star Craft 2.
My first FTS online was Star Trek Armada but that died pretty quickly.
I've played Battlefield, Battlefield Vietnam, and Batlefield 2, though I haven't touched them in a year. I used to play Rainbow 6 FPS games too way back in the day on my AMD K6-II.
All in all I think it is safe to say I like online games.