At its core, Ponk plays a lot like Dr. Mario or Tetris. You have different colored balls falling from the top of the screen, and you've got to line them up with balls of the same color to make them disappear and score points. But Ponk takes its touchscreen placement very seriously -- touch not only comes into play as you actually maneuver the pieces into place, but you can also do things like freeze the screen to move already-dropped balls around. Something like that would be tough to do with traditional controls, but on a touchscreen, you're just dragging and dropping.
There are three different gameplay modes to mess around with, and Game Center achievements and leaderboards to fight on as well. For just 99 cents on the App Store, Ponk is a satisfying puzzle/action game with a nice full feature set. Give it a shot if you want to try something new with the old dropping-block genre.