It seems I have to agree with most of the other reviewers here; the concept and execution are both great, but the game gets so difficult so quickly that you *NEED* to grind the same levels over and over again to make any progress.
There were three things that I kept wishing were different:
1 - Cheaper upgrades so the grinding portions don't take so long
2 - Shorter levels so the game doesn't take forever to play
3 - Fewer cases of "great job! Now do the same thing but with twice as many enemies with twice as much health and twice the damage." Nothing sucked more than thinking I was done, only to be sucker punched by a wave of nearly indestructible murder-bots.
So, the game is great fun, but those periods of fun came at the cost of significant periods of tedium and frustration. I hope you take these thoughts into consideration!