I really love the USB powered model, especially for something like PunchClock. No batteries means no dead batteries.
We also released a number of open source projects that can help with projects like this. Including the ibeacon cli (if you are on a mac just `brew install ibeacon`), or if you want more than iOS check out the Android iBeacon Library (https://github.com/RadiusNetworks/android-ibeacon-service).
Sorry for the shameless plug!