It's slow, ugly and it's boring. It takes takes takes and gives nothing back. You just might like it.
So why do people continue to play it?
This game punishes you at every turn, leveling is a slow, monotonous grind, the graphics are below PS1 quality, the story is weak and if you plan on having ANY kind of future with this game at all you better learn how to make some serious gil (the in-game currency) real, real quick.
I've played it for years, I'll get tired of it's bull, and quit...then I'll come back...quit again. The cycle goes on and on. I finally threw this heap of technical and graphical sadness in the garbage a while back though. But I'll never quite get over how horribly bad/fun/crappy/entertaining this game is.
Just know what you're getting into. There are far, far better mmos on the market than FFXI. XI is for hardcore RPG players and pretty much no one else. It's not a pick up and play kind of game by any means, even the most simple of tasks seem to require groups of six or more.
It has MANY low points. But it does have a few things going for it that seem to be rare in today's mmo market.
For one, the community. People for the most part are pretty nice. It takes a certain level of gamer to stick with this game for any reasonable length of time. So expect some pretty decent and mature people to play with. I personally made lots of real life friends through this game. So in that regard it's worth it.
Teamwork is another thing XI has going for it. Since just using the bathroom requires an 18 person alliance, expect a lot of fun moments when going up against big boss type characters and other huge events.
There's a lot of diversity in the different jobs (and make no mistake, they are JOBS). Each class is charmingly unique and brings a lot to the table. So it's lots of fun experimenting with the different classes. While on the subject, you don't need to create separate characters for each class. You can switch jobs in and out in your in-game house. It was a great idea at the time and still is. You connect with your character a lot more then having 93 different alts for every single little thing (looking at you WoW).
Overall, XI is a serious mess of a game, one that really just needs to die already. It's popular, and people continue to play it because of it's name, nothing more. Like I said, there's a few good spots...but the bad far outweighs the good in this case.
Perhaps FF14 will get it right.