I have one.
For background, I started off with a sportbrain years ago (mechanical pedometer sensor, used a freaking modem to upload server data), then used an original fitbit.
I bought the up because of the sleep based alarm. It works pretty well. The app is nicely done. Where it fails is the need to physically plug it in to transfer data, and the very very easy to lose plug cover. Plus I wear a watch on one wrist and got tired of having the bracelet on the other wrist all the time.
I eventually switched to a fitbit one because of the wireless sync feature. Non-adaptive sleep alarm, but being able to set the alarm wirelessly from the phone makes up for it IMHO.