If you have a computer that is slow or uses integrated graphics, don't bother buying the game. The problem is that due to poor design of the physics engine, it becomes extremely difficult when the computer doesn't meet it's performance expectations. In one case, I discovered that getting a A+ is literally impossible due to the lag on my main computer. After weeks of unsuccessful attempts, I beat it on my first try when I switched to a different computer.
Apart from technical issues, the game has a few minor design issues. The physics are inconsistent and unintuitive. For example, switching directions causes you to slide, but simply letting go of the direction key makes you stop instantly. And holding run while walljumping actually lowers your jump height, for no clear reason.
Barring technical issues, it is a difficult but fun platformer game.