Now, we're not claiming to be Tommy Lee or Dave Grohl here (heck, we wouldn't even suggest we were Phil Collins) but don't you need two sticks to play the drums properly?
I suppose you could, in theory, use two different iPhone 4s to get a more realistic experience with the Gyro Air Drums app but for the money that would cost, you may as well invest in an actual drum kit.
However, that little stumbling block doesn't take away from the fact that this is a very clever little app, that makes use of the iPhone 4's gyroscope and accelerometer to provide a semi-augmented reality style experience.
You can play drums on the upper and lower levels and move your drum-stick using your phone in a 300-degree angle. It's got a range of things you can hit like a snare, a high-hat and a cowbell and the kit stays solid whilst you move in real time.