I'm undecided about Diablo 3. Perhaps Torchlight 2 will be a better idea.
Update: Just read that you can finish the main campaign in 7 hours if you hustle (and have 3 buddies) and saw some of the screenshots (consolitis?). That's sealed the case for me not turning over my hard earned cash.
To be fair, even leaving aside the fact that (as other people have pointed out) the people who did this almost certainly were ignoring significant fractions of the game on the way by, all they did was clear the first difficulty level.
According to my understanding, Diablo III is like its immediate predecessor in that it's designed so that you complete one difficulty level, then take the same character and start the (newly unlocked) next; there are four sequential difficulties, and it's the fourth one that is intended to be Really Freakin' Hard.
Although there is, AFAIK, only one storyline, and that storyline gets repeated once per difficulty level, the overall game progression is a continuous sequence that encompasses all four difficulty levels in succession. In that sense, the people who took down Diablo in 7 hours are really only a quarter of the way in, and the hardest parts are all ahead of them.
(If I'm wrong, please don't hesitate to let me know. I'm at work right now, so I can't look up details to confirm or refute my understanding.)