Note: this blog has been migrated to Medium, with the articles here available to preserve permalinks. Please see this post at https://medium.com/@ianrbuck/duet-review-cbd3dbab630#.uorfn4581
In Duet you control the rotation of two spheres: holding down on the right rotates them clockwise, holding down on the left rotates them counterclockwise. The only task the game gives you is to avoid the white obstacles as they fall from the top of the screen. It is much easier to understand if you see it in action, so here is their trailer.
The game rewards you with achievements for each chapter you complete. More challenging achievements exist for things like completing a chapter without hitting a single obstacle. You can also comprehensively compare how well you have done to how well your friends have done.
The premium version of the game costs $3, but you can play the game for free with ads I believe. I got the premium version in a Humble Bundle, so I never played the free version. I certainly think that it is worth a few dollars, but feel free to check it out for yourself before putting your money on the table. It is available on Android, iOS, and soon it will be on Steam so I will be able to crush all my PC friends on the leaderboards.