@jamin724: This game requires no where near the commitment XI did. In fact, quite a bit of it is soloable. You can do most quests by yourself as you level to 50, with only now and then having to do a dungeon. When those pop up you can queue up to be automatically be teamed with others to clear it. Once you've hit cap, you'll probably want to see about joining the game's equivalent of a guild. You can still queue up for a bunch solo but you'll start getting into harder content and playing with people who you know can help.
As Steadyingmeat said, people can get pretty nasty at 50. Another reason to find a cool group to play with.
As for its longevity, that really depends on you. I still have fun with it but have slowed down on it a bit recently until the new patch comes later this month with more content. I should point out that there's still plenty for me to do, I just don't like playing as anything other than tanks so leveling other classes isn't really for me. I can say that for me it's definitely something I'll keep coming back to, even if I take a break here and there.
Finally, know that this plays mostly like a fairly standard MMO. I don't say that as a slight (as I said I've had quite a lot of fun with it and have been playing since launch), but just to point out that it's not as different a game as XI.