After experimenting with World of Warcraft for a couple of months, I ultimately cancelled the account. It was interesting to explore the "massive multiplayer" environment, but even with an enormous 3D world to explore, and numerous ways to interact with other players (and computer generated characters), that world becomes a repetitive excercise in endless quests and battles against enemies. For kids with time to spare, I can see how engaging it might be, but without that time, it's just an endless time-sucking monster.
My friend Andrew Tarr (with whom I created "What Ever Happened to Andy Tark"), writes a blog about Second Life, which is another take on the massive multiplayer world. I might try that shortly - Second Life is an attempt to expand these alternative online worlds to reach a wider audience. It's not so much a game as it is a world largely generated by users - including retail vendors hoping to be on the ground floor of a new marketing bonanza.
Ultimately, I think that all of these worlds will increase in popularity in a massive way when individuals can virtually enter highly defined environments, and "wander" in a fully immersive world, walking and talking with users around the world. There was a time when such a concept was pure science fiction (as in the film "Total Recall," but technology is moving ahead so rapidly, I suspect such an world can't be that far ahead.