This is a fantastic game. From the overall design, to the smooth execution, to the shiny polish--well done, top to bottom.
My only real complaint is that the last level drags on too long. I'd suggest cutting out some of the intermediate "waves" that really don't do much but lengthen the time to complete it.
The only other criticism I could offer isn't really a criticism: At the end of the game, I felt like there were so many more possibilities left to be explored within this framework. But that's a good thing, really; it means you've got plenty of material for a sequel!