Good game.
As has been said in some of the other reviews, this game is a bit difficult at first. I have but two suggestions to make:
First, try to vary the music a bit in your future game efforts; while the same repetitive Garage Band loop is interesting the first few times around, it becomes rater boring after about five times.
Second, when a player is actually playing a level, it does not count the total amount of metal earned in the upgrade screen, but instead counts only the metal earned in said level. I think it would be nice to instead count the total metal, thereby making it possible for a player to purchase an upgrade that they've been grinding for in-gams as opposed to having to wait until the end of the level to buy it.
Thanks for the game!
-Shorty