Tabata Interval Training Countdown Timer is excellent for any sort of interval training. With a few additional features listed below such as rep counter and celebrity impersonator countdowns it puts this app above most other and you'll have all the incentive you need to finish your workout.

This app will auto play your iOS library in the background and can be paused, skip or rewind tracks. For use with the reps though you need something like an armband strap which you can find at most phone shops or online to attach your Iphone to your arm. You can also change the axis for maximum accuracy.

Features
*Choice between timed and rep rounds
*Easy to change timers
*Celebrity impersonator to count you down
*Pause and resume from where you paused
*Choice of voices, male and female
*Bell to signal completed reps
*Easy to adapt for different exercise reps
*Auto plays your own music library in background
*Pause sounds, skip, rewind and play buttons
*Change axis to reps for more accuracy