I recently bought a GYMBOSS interval timer to use when I am training my clients and I can safely say its the best bit of kit that I have!!
The GYMBOSS is a repeating interval timer that can also be used as a stopwatch. The small piece of kit is easy to use and allows me to concentrate on correcting my clients technique and providing encouragement throughout the training session. I purchased my timer from Amazon for about £14 and it comes in a range of colours.
The GYMBOSS can be set up to beep or vibrate at the start/end of each interval. I prefer the beep as my clients know when to move on to the next exercise. The timer does show the time counting down between intervals so I can keep clients informed of how long they have left before they move on. You can also have two sets of intervals. I use this function for rest and work intervals.
The GYMBOSS looks smart, is easy to operate but most importantly stops my clients from cheating by distracting me between sets!!!!!