I have one - use it with my iPod. Used it when I first got it. But the thrill faded after a while. You have to remember it - my iPod is plugged into my car so when I get home I have to remember to take it out. If I was a runner and not someone running to help my tennis - and stay healthy - I'd probably still be using it.
In general, all these watches, bracelets, etc for tracking your workouts are motivational tools; not very accurate but give those who aren't really keen on working out something to use to gauge their progress. And if you're serious about your tennis and staying in shape, you don't need one.
"I jump in it!" Russian Billionaire, DirecTV Commercial...