I originally picked up Super Hexagon in a HB Android bundle. As others have said, it looks simple and has a simple idea behind it, but it's actually quite challenging, testing your reflexes and ability to recognize and observe patterns. It also has some rhythm elements to it and the music is fantastic and worth buying as well. It is more suited to playing on a mobile platform, as it's a good time killer and has smooth touch controls, but the desktop version is really not inferior at all. It's simply up to your preference.
