I don't see why people compare Guild Wars and WoW so much. They're not really the same game at all.
WoW is definitly the best MMO on the market. It's just... better, than every other MMO I've played. And I've played quite a few.
That being said, I realized after playing WoW that I'm not a big fan of MMO's in general. I just don't have the devotion to sink hours and hours into one game. Not to say that I don't game a lot, but I've found I get better return on other games.
Some advice to would-be WoWers, though: unless you like ganking lowbies, do yourself a favor and join a PvE (no world PvP) server. I played PvP server, thinking it would be cool to experience the conflict of horde vs alliance, etc. And there were a few (very few) instances of me really enjoying it; running into people your level in the same area, battling it out, sometimes winning, sometimes losing. That was pretty fun. But the majority of world PvP you get on a PvP server is just getting ganked (killed) by players much higher level than you. It's retarded. So yeah, I'd say, go PvE.
I told him once or twice; to stop playin' cards, and shootin' dice.... he's in the jailhouse now