Not bad, but too random.
As the title says, this game is pretty dang good. The only real problem with it, after playing it a few times, is that whatever routine you're using to create enemies is too random. A couple games I've nailed every enemy there, but they were few enough and far-enough between I couldn't get a combo, let alone enough points to win. Next game, using the same exact pattern of purchases, I'd get so many on screen I had to work to get my tank down in time.