I did Final Fantasy XI for about a year and a half and it's hella addicting. About the only majorly bad thing about it, is once past level 10, you MUST party to advance. It can take anywhere from 5 minutes to 3 hours to get a party, which can be frustrating at times, especially if you play at off-peak hours. I loved that you could be any job with one character though. Just gotta stop by your "moghouse" and you can change jobs (providing you have them unlocked).
I ended up quitting, though, because it's a major time sink, and I don't really have the time to play that much anymore. On top of which, grouping requires your full attention while doing so, so the significant other can get a little POed when you ignore her.
The one that I'm playing now , I've been a part of for 13 years. It's very very old school -- all text based. It's Gemstone IV. You can do most things by yourself, so it's much easier to play in spurts. The annoying thing is however, you kill things to get your mind "full" and then must head back to town to absorb it at a good rate (about 20 minutes). Advancement is very slow too. My highest is level 63 and has about 3.75 million experience. About the quickest you can gain experience is 1.7k per hour, so you can do the math to figure out how long it takes. They did add a bonus though, for the first six hours of each week, your first three hours, your experience is tripled, and the next three it's doubled. So, potentially, you can gain about 25k experience in the first six hours of every week, which makes it much more tolerable.
If only I could focus on one character long enough to get them somewhere. (63 Sorcerer, 48 Rogue, 40 Cleric, 26 Warrior, 20 Wizard, 21 Empath)... Yeah.
http://www.play.net/gs4 if you're interested. Anyone (which I'm doubting) who plays here?