I've been playing MMOs since Dark Age of Camelot. I've experienced Final Fantasy XI, World of Warcraft, City of Heroes, Lord of the Rings Online, Dungeons & Dragons Online, Champions Online, Star Trek Online, The Old Republic, The Secret World, and a few others that I can't rightfully remember off the top of my head. Some are good and others I wouldn't touch with a ten-foot polearm anymore (I'm looking at you, STO and CO). The one main reason for me to check out an MMO is that friends are playing it. If they aren't, then the game needs to have a damn good hook to get me interested.
I prefer the single-player experience, but still enjoy playing with others online from time to time.