I saw that the Elder Scrolls is going that route. It sounds cool and I like teh trailer but I don't know if it's worth a $15 monthly fee. I prefer to just play Skyrim by myself. It's really only fun when you play the game with someone you know anywya. My dad and I used to play Diablo (the original one).
We played Neverwinter and it was terrible. It had such promise but you really couldn't get anywhere without paying real money. We haven't tried Aion, but we are playing Final Fantasy 14 at the moment. That one is a subscription, but at this point I'd rather pay for the subscription and get the whole game. When we get tired of this one, we'll maybe try Aion.