I picked this up on Tuesday and co-op played with my buddy last night. It starts pretty slow - at the beginning we were dying on the second level, occasionally making it to the third.
But by the end of the session we had cleared 4 bosses (20 levels) and died to a mass of elite enemies while wailing away with max upgraded weapons.
The genre doesn't have a ton of play longevity to me, but I will definitely get my money's worth out of it. We won't level up every class to max and try to find every weapon and gear but it certainly is fun to play and a really good balance of difficulty and progression.
I will say that I am disappoint that there is a fucking DLC already.
Edit - I wonder if they didn't give Szeth some incentives to review ($$) because an in-game billboard says "hey hey people". And his videos really do boost indie sales.