Final Fantasy XI is a great game and it's my favorite MMO but should you get it? Probably not.
It's a *very* hard game to start. I haven't played it for nearly two years but I'd imagine everyone still expects you to have lots of gil (if you're character doesn't have millions of gil worth of equipment plus food, you suck) and have perfect gear. If you're new and don't have a subjob a lot of people will just ignore you. After you get your subjob, you'll still have crap equipment and then people will just think you suck.
Getting money in the game isn't a huge problem but getting enough money to make the snobs happy at low level is impossible unless you buy it.
Also, the game is very end game oriented (though this may have changed in the last two years). You spend an extremely long time getting your character to max level and it is a very long grind. FFXI is not a game like WoW where you can casually get to max level in a month or two, it will take you a year or more to get to the max level and it's just a really long grind. Personally, I like the game because I don't mind the grind, but just a warning that you will grind and grind a lot.
Finally, you're expected to play your job perfectly. When I last played it wasn't a large community and I would imagine it only gets smaller. If you were a bad player your name quickly spread across the server and people won't play with you if you're bad. People will probably forgive you because you're new but just another warning.
It's sad that such an awesome game is so hard to get into now.