This title was first released on the Playstation 2 a handful of years ago, its a fairly solid game, and is now available on the PC and Xbox 360 formats. Although it is unplayable on the Playstation 3 I have heard.
When the game was released and even up to this current point in time it is a very solid MMO, but is notably one of(If not) the hardest MMO's in terms of time sinks and difficulty.
The game has been out for quite a long time for an MMO and currently boasts 500,000 or so subscribers, which compared to World of Warcraft may not seem like much but, Pre-WoW it was massive. Even this far into the games life cycle it still boasts semi-frequent updates and a large population base.
If your not currently playing it, I wouldn't really recommend just now jumping in, you will be at a severe disadvantage and Square-Enix is actually supposed to release more information on there next Final Fantasy MMO at this years E3.
Please note: In order to play this game, you must have a Hard Drive, an active internet connection(Broadband in order to connect to Xbox Live) and pay a monthly recurring subscription fee(12.95+).