This multi-function device tracks steps and your general activity level, and syncs with your iOS device (no other smartphone platforms yet, unfortunately). But the UP also tracks your sleep, which is one of the main reasons I've been waiting for it to be released.
The Jawbone UP is $139 which seems like a good price for the functionality it offers.
For what it's worth, this is the second version of the UP - the first version was released a year ago and discontinued because they kept failing. The new version seems solid based on my experience so far.
My impressions of the Jawbone UP
Comfort: The UP band is very comfortable. I wear the Large size, and it fits very well (I used Jawbone's sizing guide on their website to figure out what size to order). The band is flexible and coated in a soft rubbery finish, so I barely notice it's there, and I wear it all the time except when showering (it's water resistant, but I'd rather avoid getting it wet when I can).
Battery life is very good; you charge it using a special USB adapter provided with the UP and it takes about 80 minutes to reach a full charge. Once it's fully charged, the UP lasts for about 8-10 days before it begins giving you the "low battery" warning.
Accuracy is decent, when it comes to exercise tracking. I've done some testing of the band by counting my steps and comparing the steps it has recorded, and it is typically a bit low (conservative). However, in the few tests I've done, its number is 95-98% accurate (I notice it is less accurate when I'm carrying something with my left hand, as I have the UP band on my left wrist). I'd much rather have it under-count my steps than over-count them. Sleep tracking, by the way, seems spot-on.
Syncing is pretty easy. You start the app on your phone, remove the plug cover and plug the UP band into your phone's headphone jack. It takes a few seconds to load the data into your phone, then you can unplug the band, replace the cover and put the UP back on your wrist.
The App not only gathers your data, it also uploads your stats to the cloud to maintain a historical record of your activity.
Minimal, easy controls make the UP a snap to use. There is only one button on the UP - a silver push-button on one end of the strap. If you tap the button, you can tell whether the band is in "Sleep mode" or "Awake mode," through a small LED display on the edge of the band - you see a moon in Sleep mode, and a Sun in awake mode.
To toggle the modes, you "long press" the button, and it will vibrate to let you know it is changing modes. You'll also see the LED indicator toggle the Sun and Moon indicators when you long-press the button.
Sleep Tracking