I've been playing World of Warcraft for just one month now, and it's extremely addictive. I'm on a PvP server (Stormreaver) as a Hunter, and I actually find the play quite a bit more strategical than Final Fantasy XI, I don't know if it's just the class I picked or what; but when you're minding your own business and you're suddenly jumped by Horde, you need some quick thinking and good use of abilities to get out of the fight alive as it is a very fast paced game and basically any single action you take/don't take can determine your fate.
If I remember correctly, after one month of Final Fantasy XI, I was just about getting sick of the constant need for parties, but I dealt with it, as boring as it was, simply because I wanted to achieve something. Starting as a Warrior, I could go into battle, auto attack and Provoke every 30 seconds and that'd be it. This was the case until probably level 20's, where Skillchains came into play. Looking back, I only now realise how boring this was. The simple fact that I had something to aim for kept me going though, up until now, 61 Paladin, I'm very bored. What is left for me to do, seems really chore-ish, and I'm not up for any of it.
I'm not saying either game is better than the other. Final Fantasy XI seems to be aimed towards the more hardcore gamer, whereas World of Warcraft can appeal to any gamer. You really can't bash either one of them as they are both amazing games. You only have to check out their award pages for proof.