I have the Polar M51 for Cathe workouts for like three years now. And I recently spent a lot of money on the Polar625X for running. I love them both. It helps me keep taps on my progress and w/o intensity. It tells me how many cals I burned (thus how many chocolates I can eat), whether I trained in my zone (or not) and if used properly, the M51 gives you enough info to notice overtraining early. It also accumulates all workout time so by year-end you'll know how many hours you spent working out and totals your cals over the year. I really wouldn't want to workout without one anymore. It can be a great "coach".
If you get one, make sure that you know which features you want, so you don't buy the very expensive ones with features you'll never use.
Dutchie