I set it up, myself. I did notice that if you've already used Google Voice (non Sprint integrated), I didn't see any way to re-setup with your actual Sprint number.
The service works well, as you'd expect. Features work as you'd expect - visual voicemail (of course, providing the person spoke clearly enough), you can play your voicemails through Google Voice, as well as being able to send and recieve text messages with your own number (the messages YOU send on the phone will automatically show up on Google Voice, which is neat).
I'm not entirely sure I understand how actual calling works - haha. I don't know if it automatically makes calls through your google voice number or from your sprint number, nor do I see a way to tell (as your number and the Google Voice number are the same). I would imagine that it uses your own number, since there's no delay in making calls where the phone would be connecting to GV, which then connects to the number you're calling.
My question, then, is: Do you lose call quality by having it connect through GV, should I be using the GV connection instead of my own?
I don't find the need to use an app on my phone, but I believe the one I have is called FreeTalk (Could be wrong - will update in a moment when I can check my phone).
Hope this helps!