Artwork-wise, I like the pixel style, so have nothing to say there. I didn't have any problem with the controls either, but only found you could shoot and walk backwards by accident - makes it a little easier.
I don't think hard mode is hard enough. Perhaps it was because I'd had to complete easy mode first, but it wasn't at all difficult to complete, especially when double jump is the reward for the first level. Some of the bosses were complete pains on either mode, though - the second boss has the ability to kill you if you get hit once, as that sends you flying into the air, ready to hit him a few more times, taking damage every hit. The final robot was a test of patience on easy mode as the fireballs destroy the platforms seemingly according to luck at first - you have to be lucky enough to get up to hit him, which was pretty annoying.
Overall, though, it was a fun game. I came back after I'd beaten it, which is always a plus :).