Square invalidates your serials after three months. You'll have to buy a new copy of the game to get serials that will work. If it's been more than three months, your characters are gone too. Yes, completely retarded.
I played FFXI for a couple months after NA PC launch, gave it up for about 4 months, came back and my characters were gone and serial numbers invalid. It's spelled out in the manual, but it's nowhere on the outside of the box, and once you open the software you're stuck. I wasn't too bothered by it at first since I had picked up a PS2 hard drive on sale and thus had new serials to get back into the game, but then I had to give the game up again (had some personal issues a few months back that were eating up all my time). I've been tempted to start playing again, but now three months have passed, which means my characters are gone again PLUS I'd have to get new serials. This is a boneheaded move on Square's part to not at least keep your character info longer than three months. Lots of people come and go with MMOs, playing for awhile, leaving, then coming back later because they miss the game or want to check out new content (I'd like to see what Promathia added).
I'm playing WoW ATM and enjoying it, but as Belfast said I don't know yet if it will have the staying power of FFXI. Grouping was a major PITA with FFXI, but I also got a greater sense of accomplishment out of doing things in FFXI, and it was easier for me to make friends in FFXI since grouping was so necessary. However, the economy in FFXI was pretty busted by the time I left last summer; I can only imagine what shape it's in now. It was interesting to play the game again a couple months after NA PS2 launch to see how the community had changed. Not saying there was anything wrong with PS2 players, but by the time of the PS2 launch there was a ton of info about the game online. It was so much more entertaining at PC launch when far less was known about the game (only what beta players had discovered and what could be gotten from Japanese sites). Of course that's true for any MMO.