I played the demo and didn't like it. But I bought it anyway, because it's D3, and I couldn't not buy it after the countless 100's of hours I spent in D2.
having said that, I've logged 17 hours in 2 weeks, taking my good old time progressing through normal with a wizard. Do I think it's worth the $15 I paid for it (I had a gift card)? Absolutely. Would I have paid $60 for it? Probably not. 9 hours per week is terrible during the summer, while I have hours and hours of free time.
Here's my mini-review.
There's 3 main issues with the game, as I see it.
1
The auction house is a necessity, because the loot drops are so bad that you really need to spend gold to get a good weapon. But once you realize this, gridning for the best weapons that you'll never find becomes pointless. You just need to find weapons to sell, to make gold, to spend in the auction house.
2
The characters aren't dynamic. Once you have a level 60 wizard, there's no reason to roll another wizard, you can just re-spec your current one. Want to make a melee sorc? Simple: Just pop over to the auction house, buy what you need, switch some skills, and 10 minutes later you're done. There's no feeling of ownership of my character.
3
The story sucks. I wanted to play single-player, take my time, and enjoy myself. Some of the voice acting/dialogue is so bad, it reminds me that I'm not my character in the game, but just a person playing a bunch of 1's and 0's.
So lets sum this up: The game is about gold, not loot. There's no replayability in single player, and what little single player there is, sucks.
Of course, this is all assuming that you can manage to play the game at all; For that, you need an internet connection, Blizz's servers to be up (not so much an issue anymore), and a bit of luck to not have high latancy/ping. The game (IMO) is an MMO, built around the auction house (and probably PvP), that doesn't do any justice to the first 2 Diablo games.