There are people who still play it, although the amount of subscribers is on the decline with how old the game is at this point. They recently merged several serves though, which means that the serves that are left are much more populated now.
I played the game since NA release and quit near the beginning of this year due to money troubles, although I've been keeping up with a lot of its updates. I would say the game is quite a lot of fun, but you should be wary while considering buying it. Unlike a lot of recent MMOs, XI does make party-based play pretty much mandatory, as well most of the XI population at this point consists of people who have been playing for a very long time, which unfortunately means a lot of elitists. The game is also notorious for being very difficult and time-consuming, although a lot of recent updates have been working to soften this significantly. Over all, it is unfortunately not a very new player friendly game.
If you want to play, I would highly recommend trying to look through some forums to get involved in a Linkshell (guild) that is willing to assist you in being a new player as quickly as possible, otherwise your experience is at very high risk of being extremely negative.
If you have the time and patience are can past all that I would say the game is worth it. It can be a very difficult game, but it is a very unique feeling MMO being from before WoW set so many trends for the genre and possess a lot of variety, it is even quite strongly story driven if that means anything to you as well.
