Even though I'm extremely glad I stopped playing it I can't say it's a bad game.
There's no MMO even comparably as good as WOW on the market (yet). There's seemingly unlimited content (which makes it addictive), great interface and combat system that's balanced between classes like no other game, an extremely active community with custom UIs, plenty of players active and constant updates. Also, the game is so big that it could fit Guild Wars ten times over.
I don't want to advice anyone to start playing MMOs due to my own experiences, but if you absolutely have to - choose WOW. It's simply the best as Tina Turner would put it.