Depending on whether you've ever heard of Tap Tap Revenge or not will determine how much fun you're going to have with it. Before the app store launched, Tap Tap revenge was available to those who had gone through with jailbreaking their iPhone.
The app originally allowed you to use your own songs in a Dance Dance Revolution vs Guitar Hero style of gameplay. Anyone could create the beat pattern to any song they had and upload it for everyone to play if they had that exact mp3 on their iPhone. Now that the new version has come out officially, things have changed.
You can no longer use your own songs or create your own unique tracks. What you can do is play several free songs that come packaged with the game or download new songs that have been made available for download. The songs aren't anything really mainstream, but there are four different modes of difficulty for you to play through (easy, medium, hard, and extreme). It depends on your taste in music as to whether you'll enjoy this game in its current form.
The game offers both a one and two player mode. The two player mode is actually pretty enjoyable as you both hold one side of the iPhone and play at the same time by tapping the balls of light on their corresponding color line as they reach the base. You can create your own online identity as well and share your high scores with every other Tap Tap Revenge fan. This is one of the better free games out there for the iPhone, so make sure you at least give it a shot.