I think FFXI is potentially far more addictive than WoW for various reasons, though that's coming from a pretty hardcore endgame perspective. If you're worried about addiction then I'd stay far far away, but who can honestly resist, right?

The game takes time. It takes lots of time. Too play competently you'll need at least 3 hour blocks regularly. You pretty much can't get XP unless you have a significant amount of time to devote to a group. Rudely leaving often, shortly and suddenly will tarnish your reputation and make the experience less enjoyable.

The story-line is awesome, though you won't even begin to scratch the surface of it until you've got a job at 75. Getting a job to 75 will take most people well over a year and there are many who've been play for 2 or 3 without even one level 75 job.

On the bright side, you'll scarcely run out of things to do. Unlike most MMOs where you pick one job/race combo, in FFXI you pick one of 5 races and can level every single job (of 18... soon to be more) to 75.

There are a handful of painful quirks about the game. The learning curve is sharp. You MUST group to accomplish anything. I mean if you want to blow your nose you'd better put up your flag and look for a group to do so. There are less people in the middle range levels (37-70) than there once were. Certain jobs get crap to no invites and in those level ranges are impossibly slow. I recall LFG for many consecutive days in 6 hour blocks on both Thief and Dragoon with no invites. Even if you're a good party builder there's always going to be a lack of either healers or tanks.

Storage is a huge concern. With playing multiple jobs you'll be fighting it a lot. You'll end up having to have mules. SE requires you to pay a buck for each additional character. While some people manage to get away without having many (any?), most of them suffer as a result. Even if you have only one job, if you want to play it to the fullest you'll need level capped gear for 30, 50, 60, and maybe even 70, with several full sets for level 75 in various situations. You'll find yourself sending it back and forth between mules often.... 8 pieces at a time, not counting the Rare/Ex stuff you can't send which you have to find a place for.

Money is also a huge issue. It's not easy to make. If there's a reasonable way to make money... RMT OWN IT! They monopolize pretty much everything. The crafting system is in a state of utter brokenness. SE has started fighting back against RMT and cheaters (after years of apathy), which is great, but in so doing they have set the economy into a free fall of deflation as they take huge amounts of currency out of the economy each month.

You'll find that you'll have to take regular breaks from 'playing' the game to make money and deal with other junk. This is not a very casual friendly game and the amount of elitism is pretty high. It really hits hard at the top ends of the game as well. Speaking of, the game is pretty high-level centered and much less time (development-wise) is dedicated to low-mid level content.

You won't see even a fraction of all of the awesome things the game has to offer for well over a year. I played for 3 years with a play time of well over 300 days. I'm pretty much doomed to come back as there is just a certain flavor that FFXI has, despite the numerous flaws, that just draw me and my wife in.