I'm enjoying it. Played a witch doctor until, I think, just about the end of act 1. The new skill system is certainly working out for me. I like having the option of a number of skills of (relatively) equal utility available to me. Compared to previous incarnations, where I would basically sink all my points into one or two skills and use them exclusively, I now have the option to play in a number of different styles depending on my mood. I wasn't crazy about the system when I first saw it, but it's working for me.
I can't speak much to the plot, seeing as how I'm not even done the first act, but so far I'm not disappointed. Obviously, as an action-rpg, a person can't reasonably expect Hemingway-tier plot and characterization. Keeping this in mind, I find that the plot is certainly serviceable. And I actually chuckled at some of the character banter as well; the voice-actor for the Witch Doctor is surprisingly good. Admittedly, I was drinking a beer or two while playing (Driftwood Singularity, I highly recommend) so I may have been a bit more receptive of an audience.
I am really enjoying the art. The environments (so far) are beautiful and there is a real feeling of physical connection when you land blows on an opponent. The skills are (at least for the Witch Doctor) quite inventive and entertaining. Anyone who solves disputes by throwing jars full of spiders at people is good in my books.
The most obvious and aggravating problem with this game is the always-on requirement. I lack the writing skill to accurately convey how massively frustrating it is to die because of lag in a single-player game, or not be able to play at all because the servers are down. It's ridiculous! I understand why they did it, but I sure as hell don't support it. Blizzard should have found a better way to work this out, or just drop the real-money Auction House altogether. This is a fun game, and I'm going to keep playing it despite this issue. But I need to say that this one feature alone is almost a complete dealbreaker on one of the (otherwise) most fun games I have played in a long time.
@Double A; if you've never tried an ARPG before, this might not be your best choice. I'd recommend Torchlight or Torchlight 2 (coming soon). They're cheaper and simpler options than Diablo. And you won't have to deal with the always-on BS.