I've been using the smartband for three weeks now (in combo with my Xperia ZR and Tablet Z) and it works fine.Never had any problems.
Accuracy of the stepcounter seems to be not very good but for comparing between days it works just fine. Sleeptracker is an nice feature which works really well. The lifelog app can also keeps track of your app usage, even on multiple devices (in my case phone and tablet).
Only thing which I find a bit disappointing is that the smartband doesn't recognize when I'm cycling or riding a motor vehicle (which does work when using the phone sensors).
The lifelog app can be used without a smartband, using the phone sensors, but still good for testing the app. This has a quite large impact on battery but I could still make it through the day.