A number of people I know who took up WoW went from out-going, social butterflies to sun-fearing cave-dwellers upon picking up that game. They changed into different people. It was impossible to get them to do anything, as they were all coordinating raids with people across the planet in their free time and when they did manage to wrest themselves away from it, all they can talk about is the game.
So yes, I've seen what the things do to people, so when I stereotype all MMO players as dirty, unwashed, agoraphobic rejects, I at least have some grounds for it.
I have no criticisms for any MMOs though. I've tried a few, thanks to free trial periods, can see how they could be addictive and I congratulate the various MMO publishers for finding a formula that beats Piracy into a bloody pulp while making gobs and gobs of cash, but won't be making a time/financial commitment to any of them myself. Besides, I love the Sun too much.