Right, so my impressions of Diablo 3...
It's good. I can't say "great", but it's good.
The gameplay is addictive and old school, and is by far the games best selling point. It's quite ugly graphically. The cut scenes are tremendous but the actual game feels about four years old graphically, which is a disappointment on one hand, but it also lets lower spec PCs play comfortably.
The biggest mistake they've made with the game by quite some way is having an internet connection as a necessity to play the game. You have to log in and stay logged in to Battle.net to play it, which - unfortunately - has resulted in a massive amount of problems with connections, servers, lag and such. Most of which you'd expect for a launch of this magnitude if it were an MMO, but this is essentially a single player game with some sort of PVP in the future. But the easiest thing to do would have been to do it like Warcraft 3 - have the single player offline, sign up to Battle.net to play PVP. Simples.
So for something quite trivial, Blizzard have messed up the launch. In the modern era, when competing against absolute classics like Skyrim for the playing time of a gamer, Diablo 3 doesn't do enough to compete consistently. You'll pick it up, play it for a few hours but then get back to FIFA, Football Manager, Skyrim, COD or whatever else you're addicted to. That said, Nightmare Mode will truly be hellishly difficult and it will take the more determined player a long time to finish.
Which, of course, you'll need to spend more real life cash to do due to the God-awful idea of adding a real world Auction House in the game. Casual players will complete the game easily on normal difficulty, but a glass ceiling will be there.
So it's a 7/10 for me. I wouldn't put a priority on it, and I certainly wouldn't splash £40 on it, but at the same time it's not going to massively disappoint you if you did.