I liked the idea, and the way it all looks, but was confused by quite a few things.
What were the grades for at the end of each level? At first I assumed an A was for passing on first try, or for winning without losing any monsters, but neither of those seemed to be right...
Then when I won the last level I got a B?? Where did that come from?
Since there's nothing for the player to do during battles, Endless mode seems a bit pointless. Thankfully it isn't 'Endless' at all (I'm assuming it stops after the 5 minute timer runs out - only played it once, died at level 35 with about 30 secs remaining.)
Basically, it seemed pretty confusing at first; then I played 5 or 6 ! missions to figure out what I was doing and make some cash; then the rest of the game was over in minutes. It was fun for a little while and could be expanded into something really good.