Spoilers
In a lot of ways, this game is like Diablo II, in terms of how you progress.
So, obviously, act i and ii take place in the same areas in Diablo II. Belial is stupidly hard, probably the hardest boss fight in the game, same as Duriel back in D2. You travel to act iii by speaking to Tyrael. Azmodan is stupidly easy, just like Mephisto. You travel to act iv by following Diablo through a red portal, just like D2.
I thought these similarities were funny.
Anyhow, I've had fun with the game, been playing Demon Hunter. I soloed everything, didn't use mercenaries. Diablo was a little easy in my opinion, Belial was without a doubt the hardest and also most fun boss. The act progression in iii and iv was a little fast, almost too fast for my taste. I did both of them in one sitting, simply because there wasn't any downtime ever, you could just keep on going and going.
Overall, the game isn't particularly better than Diablo II. I'd say it is slightly better in some areas, for example items, skills and monsters, but the story is lacking and the graphics are a little disappointing at times. You never really got that feeling of belonging like you did in Diablo II. The Diablo II towns had a lot more spirit, is what I'm saying I guess.
Buy the game if you liked Diablo II. You won't be disappointed.