The app is super useful also. You can Arm and Disarm and basically everything else but from your shooting position. From prone or sometimes certain light conditions you cannot see the screen and its really handy to be able to use it and view information without breaking your shooting position.
Good call on the app. My only complaint with mine is the UI can be weird if you don't use the device a bunch. I will try the app next time.
If your just going to check a few velocity the AA battery are fine but if you want to use it for long sessions you will need a battery pack.