I played Dark Age of Camelot on and off for a couple of years. It's really a whole different way of gaming. You have to really keep an eye on whether or not you're having fun with it -- sounds odd, but it's true that it becomes a bit like a job. Or kind of like filming a movie with lots of hanging around waiting for things to come together so something can happen. The people who enjoy MMORPG's the most, I think, are those who enjoy chatting with people during the downtimes, or while grinding away at levels.
DAoC had the advantage of real player v. player (i.e. realm vs. realm) three-cornered battle. And once they got going, they were indeed a lot of fun. The questing and level grind were not always fun. They were in the beginning when it was a goal in itself and people actually role played their characters some. More recently, people are moving through the leveling as fast as possible and new players are kind of wondering how to get into the game.
I have heard WoW is more fun for the PvE aspect, but PvP is not much to write home about. I'm not going to check it out as I just don't want to spend that kind of time any more. Even as a somewhat casual player, I probably played 10 hours a week or so -- a lot. The fee does not seem that bad, especially for the people who play a whole lot, it becomes pennies per hour.
When you're into it, the thought of playing a single-player game doesn't appeal much because it doesn't seem to *count* somehow. But, eventually, you get disgusted with yourself and snap out of it.
I am fooling around with the open beta for the Japanese server of DAoC, but that is mostly just to see if it's a good way to practice my Japanese.