I'm having fun.
I can't wait to get home from work each day to play, even if I only play for an hour or two (it's usually longer).
That's all a game requires for me to feel it's "successful". I don't care how well it sold, I don't care how much money its publisher made (or makes).
Is Diablo 3 fun? For me, yes.
Thus, it's a success.
because you're so important so if something is fun for you, it is sucessful