Great game. Enemy&boss designs are awesome(and I actually mean it!), physics effects are actually useful, music fits the gameplay. Most of the problems come from the fact that it was made with flash. Controls are somewhat laggy, framerate seems to be capped at 30, but still doesn't feel smooth, bad gamepad support, etc. None of these problems are game-breaking though, just a minor irritation, for me at least.
Update. I see that many reviewers compare it with Contra/Metal Slug and don't like the added physics effects. Well, this is NOT Contra. To me, its more like Abuse, which also was slower, controlled with mouse and involved slightly more thinking than average platform shooter. Physics, in my opinion, adds to the game, making every playthrough slightly unique, creating unexpected opportunities as well as irritations. I heard, this is called "emergent gameplay", but Nintendo kids seem to prefer games they can memorize and play by reflexes alone, instead of improvising, even a little.