Awesome game, reminiscent of Contra (minus the directional shooting, as well as being able to drop through platforms, which would be nice). Nice pacing and feel to the game.
One complaint-- in the first section of Level 4, you have to jump onto and from the falling box in order to get to the next platform, and this is so remarkably inconsistent and hard to pull off with the current game mechanics. Making the next platform just a little bit lower, so the success rate is tweaked slightly up, would be a nice revision.
Also, thanks for not making this true-NES-style and giving us only one or two continues with no passwords...