Good!
I liked this, a good idea (yay Metal Storm!) and implemented well. It's not immediately obvious that you can use the mouse to do stuff...I didn't see that mentioned in the help, but it was mentioned in the in-game tutorial, but I guess it's okay. There were just two things that bothered me.
1) The controls change when gravity changes. If gravity is left, then suddenly left is jump. It's hard to wrap your mind around. We're much more used to playing games where the controls stay the same but the direction of play changes.
2) Why can't you change gravity in midair? That was one of the really cool features of Metal Storm.
Anyway, good job!