This is a MUST HAVE for exercise enthusiasts. It saves so much time in your workouts, as it makes sure you push really hard through your exercises and take schedules breaks - which is great as often even when you might say you'll take a 30 second rest, it might turn into 60 or longer, so having this timer is absolutely marvellous for making sure you get the most out of your workout in a short period of time.
It has 4 options for keeping track of time: Stopwatch; Countdown timer; 1 Interval; or 2 intervals. This means you're always against the clock, which psychologically is great for pushing yourself to your limits and skipping unnecessary breaks. You can change the intervals and countdown timer to whatever time frames you like, so depending on the intensity of the exercises you are doing, you can adjust the length of time you do the exercise and length of rests you might need.
The design is also great, at first programming the thing is a big of a faff, but you'll soon figure out how to do it and then it is really easy to change up your intervals if you're doing more than one part to your workout. Also the clip on the back is very strong, it's unlikely to fall off of your trouser waist or where ever else you might clip it on to. To alert you of the next interval, you can set it to beep high or low, and/or vibrate. The vibrating option is good for the gym if you don't want everybody else to hear the beep, but still keeps you on track of your workout.
I've had my timer for little less than a year and it is a absolute must have if you want a short intense workouts which will give you all the benefits of a longer workout.
Highly recommend.