The game is not bad at all, but it suffers from things already mentioned by others.
-jump upgrades should have been double and triple jump, where you can press the up key again in mid-air to jump an extra time. Then you don't lose the advantages of a single jump
-I didn't know how to use the coins for upgrades until I entered the second star system after finishing all first star system levels.
-It's annoying that if you finish a planet you're in the star system selection screen instead of the screen of that planet
-Some levels are really easy, others too difficult (it starts quite difficult, then after your first upgrades things get really easy, then the final levels are extremely difficult again and require constant grinding in the same low levels for coins).