Pretty good but some improvements could be made, like for example that there is no menu or Pause button. Still by far my biggest complaint is the jumping. Controlling Malio in the air is just awful. it leads to unnecessary difficulty, that doesn't depend so much on the ability on the player, but more on the fact that the controls suck. Being that jumping is the main thing in this game it should me much smoother and controlled. Not only is the movement in the air absurd, but you also loose impulse if you touch a wall; you also jump way to the right when riding a bullet... stuff like that.
Although in the end it is not bad, I got bored after grinding a bunch of levels and bosses and didn't see an incentive to finish the game.