One word: Terrible.
I own the game and have played it to the Final Act (Which is where I stopped... right before the final boss). Acts 1-3 are very bland. It's the same 5-6 monsters and the same 3 environments; subway, city, or sewer. In Act 4 you get a two new environments, but still the same 5-6 monsters. Act 5... gets a little innovative. Some new game play... no new monsters... and a decently cool cinematic before the end.
Now - What really makes the game bad? Constant server errors, memory leaks, and buggy game play. Just cleared mobs for 20 minutes straight, almost to a boss... SERVER ERROR! You now have to re-clear everything you just killed because the server doesn't save your progress like that.
Also, the game just feels unfinished. There are no item sets currently in the game... wtf? If you play the game through as a melee class, you'll probably be extremely pissed at the final act. I'll try not to spoil it, but you have to clear a gauntlet... and one of the bosses flys, the only problem is that he never comes down close enough for you to hit him. So you basically stand there like an idiot and pray that he comes close enough to the ground to hit at some point. This game still is in beta. So I recommend that you give it a year or so to patch up and then buy it.
*edit* I should also add that I recently downloaded the latest .6 patch to try and finish off the last boss and at least beat the game. After loading and clearing to the boss, I received a server error. *sigh*