I'm pretty much done with Diablo III for now, I've played it enough to conclude that it's basically a single-player MMO with about 10 hours of unique game content, and one quest line. The freedom to do everything has stripped me of the will to do anything, because nothing feels special.
Co-op is fun, because playing with friends is always fun, but there is nothing particularly fantastic about the Diablo III experience compared to other games.
This is not the impression of the game I expected going in, I had very high hopes and was quite a defender of the game against internet whiners.
Not a terrible game by any stretch, and I enjoyed playing it. Just nothing special.