Good game, but could have been better.
I didn't like that the upgrades I bought did not apply when I went back to do the challenges. In other games as you progress through the story and earn power ups the lower level challenges get easier because power ups still apply. Also, if you fail a level you do not get anything from it. Meaning, if I spend ten minutes getting through a level and die at the end, that time is wasted. There should be a system that you get points for killing bad guys, and so many point earns you a star.
The above factors about this game make it especially difficult if you get stuck. Replaying lower levels does not benefit you, and the challenges are so difficult they are not even worth trying more than a couple times. I beat the last level, and I would like to unlock the hidden levels, but it would be too time consuming. All in all, it was a very fun game; some may think I a am a wimp, but I think it was too hard.