The Amiigo.com currently in development is the one to look out for. It is designed for swimming, aerobics, gym/cross training and various other activities. Measures your heart rate and body temp. Can't wait.
The M7 chip works great, when the phone is in your pocket in your jeans.
However its not working when i have the phone in my backpack, or in my jacket pocket it simply track 0 steps taken - even in my hand.
also when at home or at work I often leave phone on desk.
Being placed on the wrist it might be better at tracking movement than a phone in pocket (or even a purse). Best is strapped to your waist for which the Fitbit one is perfect (some do strap their phone to their waist but most don't).
Certainly better for measuring the heart rate (phone has zero chance but wrist-based devices are also not terribly good at that).
Tracking sleep is also easier, at least it doesn't require the user to do much, compared with placing the phone under your sheets every night.
It has an altimeter, depending on where you live and what you do that can a very important element in measuring activity.
For things like heart rate and altitude, being able to see them on your wrist can be very useful.
So, it's really its location that offers advantages in a number of areas over the iPhone.