This is one of the best games I've ever tried. Seriously.
But if you expect something like LOTRO, WoW, EQ2 or anything similar, then stay away. For example, for leveling and other ways of fighting monsters, there are almost no solo options (after lvl 12 or so), you WILL need to party/group with other players to progress.
FFXI is not a rush to max lvl, then raid only, as most of the newer games. This is much more slow-paced, and for the high-lvl activities (not only "end-game"), this game has more options than any other games out there. It can be casual-friendly, but not solo-friendly, not even close.
The community in this game, is one of the most mature out there. If you happen to be one of the kiddies from WoW (ie. begging for money, or spamming LOL, OMG, etc all the time), you won't stand a chance. If you behave, you will see that nicer and more friendly gamers doesn't exist elsewhere. The player base is very stable, it's been around 500,000 for a very long time, according to the official census.
Most MMORPGs are very time consuming, but FFXI is even "worse" than most. If you expect to be able to log on for half an hour to do a quest or lvl a bit, then forget it. FFXI is for players who are able/willing to play long sessions, that's just how it is.
FFXI doesn't have region specific servers, like most other games. Each and every server has players from all over the world. This is really great, cause no matter what time you log on, the servers are never "dead".