I played FFXI for a pretty damn long time. If you're a patient person, it's a fairly fun game.
The first 10 levels will be you soloing for the most part. Around level 10-12, you head out and start partying, which is the primary method of gaining EXP from then on, unless you happen to be a Red Mage or Beastmaster (have to be level 30 with one job to unlock Beastmaster). If you do decide to solo, it'll be slower than partying. Leveling is a pain sometimes and you'll occasionally find yourself looking for a party for a long time, unless you happen to be one of the more necessary classes (White/Red/Black Mage, Bard, Paladin, and occasionally Thief). However, this isn't really an issue until you start getting into the later levels (around 35-40-ish). I've had times when I'd be on for two hours looking for a group without any luck (I was a Dragoon and they were considered gimp back then). In times like these, I'd just jump on my other computer and work on my game or play something.
FFXI has two things that most MMOs don't have. First of all, it has a pretty good story for an MMO. It starts of somewhat slow, but it picks up once you get rank 3 and start doing those missions. Cutscenes become pretty cool as well, something which most MMOs don't even have.
The second thing FFXI has is a great community. Unlike WoW or most other MMOs, adults tend to play XI more often than kids. This means more mature players, which results in a more pleasant experience. And although leveling can get tedious at times, the fact that you'll have 5 fairly intelligent and (usually) sociable people with you means that it isn't all that bad.
The graphics in XI are very good compared to most MMOs, and that's a plus (I personally can't stand WoW's or any other 3D MMO's graphics after playing XI). There are actually a lot of places to explore in XI but most of the stuff you find (occasionally you'll find stuff like goblin tracks, artifacts, etc.) isn't accessible until the later levels, so while there are many places to explore, you probably won't be doing much interaction early on.
It is a grindfest at times so if you want instant gratification or don't have much time to devote to playing (you'll usually be in one party for 1-4 hours), games like WoW are a better alternative since they involve much less grinding. I'm currently playing WoW, but if I had more time to waste (college student working part time), I'd definitely be back playing XI instead.
Most people are turned off by the time you have to devote to it, so if you're like most people, you won't like it much. If you do decide to start playing, join a linkshell (the guilds of FFXI). Playing by yourself without much interaction gets boring very fast (this is true of any MMO) and is the reason I quit playing XI the first time. Having people to talk to will make the experience much better and you'll be able to get help for quests and missions. Make sure you pick a job you'll like to be your main. I did Dragoon just because I loved their jumps, wyverns, and fairly nice burst damage.
@DS: About the last boat comment...you could always pick up fishing and fish while you're on the boat. Boats sometimes get attacked too, so there's always the option to fight the monsters off (usually not possible at lower levels though). Plus, once you hit rank 4 (or maybe 5, can't remember), you get to ride in airships and they have a much shorter wait and take only 3 minutes to ride. And the reason the game gets more fun past the 30's is that the story becomes more interesting, the more childish players stop playing, you start to get a bunch of good abilities/gear, battles become more challenging and require teamwork and strategy instead of the standard auto-attack, and you get to start doing BCNMs (which are tough little boss fights with pretty good rewards). PvP also opens up (which is basically like a sport with killing) and that's one of the best aspects of the game IMO.
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