Well it's been out for longer than a year and a half, more like two and half to three years (if you include Japanese release). I had been playing it for around 7 months and then I left for 3 months to play WoW.

I am still interested in FFXI so I came back to it a week ago. I believe there is still enough interest in it to keep it going extremely strong. If you don't like MMORPG's and get disinterested in them that easily, then I suggest not ever playing one again.