I haven't played it yet, but everyone I know personally has liked it when they tried it. Even the guy who didn't like Diablo 2 back in the day said he was impressed with how well done the game is.
Subjective as always. All my friends from university I am still in touch with who played Diablo 1 & 2 can't stand Diablo 3, myself included. It's pretty novel seeing Blizzard finally release a bad game, but also disappointing. The game just has a lot of production value behind it, but none of the substance of the past games.
Blizzard's trouble is highlighted in some of the earlier discussion in this thread. The departure of Blizzard North gutted Blizzard of the core talent that contained a large portion of the group of people who created their successful franchises; Warcraft, Starcraft, Diablo and World of Warcraft.
The Blizzard North team was in large part responsible for the creation of all these franchises. Blizzard has been riding and iterating on their work and making bank off it ever since. They haven't made a new IP since. Diablo 3 is very close to the nearest thing they've done to a brand new game since then. It's quite different from the past games, and the gutting of their core creative talent shows in the poor quality of the game, imo.
I doubt their unnanounced MMO, Titan, will ever live up to the quality of their past creations. They've lost the people who were in large part the creators of all Blizzard's success.
Diablo 3 while impressive in release week sales, is a pretty lackluster title, release day sales were more an indication of the value of the Blizzard brand and the notoriety of the Diablo franchise. If you check out user reviews and popular gaming forums, including Blizzard's own forums, it is far and away the worst received game they've ever released. I think they are in big trouble in the talent department and are going to have to get creative when the well dries up of milking the same franchises they have been for the past 14 years.
The merger with Activision is not doing anything to help the quality of their games either. This is a company that aims to maximize profits, often to the point of bleeding the proverbial franchise cow dry and having to find another to kill off - see Guitar Hero and Tony Hawk.
People shouldn't have illusions about how things work now with Activision/Blizzard. Morheime, Blizzard's CEO, reports directly to Kotick. Kotick calls the shots. Activision behaves like a hit and run organization in gaming, they swoop in and monetize a successful franchise and milk it dry. That can't remain viable forever and they are vying against EA, who is out to do the exact same thing.