I've previously used a GymBoss for timing intervals when performing normal circuit training or HIIT which was good, but it was limited in how you could configure the intervals. Then I came across an iOS app called FlexiTimer, which is so much better. You can create as many timers you want and each timer can have as as many intervals you want, with different alerts and music for each interval. The music feature is great if you like different music during high intensity and low intensity intervals. You can also use it for timing running programs like Couch25K.

It's a little on the expensive side for an app at $6.99, but that's a bargain compared to a GymBoss and it's so much more flexible.

Website: http://flexitimer.com

It's on sale at $4.99 for a couple of weeks