I setup my DI2 on my PC, but you can do it on your phone now with this and the new battery. I noticed battery life has gone down a lot so I am now only planning to use this if I have any adjustments to do on the ride. Nice thing is if you have issues with the DI2 on a ride, you can access the system with your phone to make adjustments if needed, or get out of limp home mode. But, its not an essential piece of gear for DI2, in my view, so 4 stars.
Super small and light, works great inside my carbon frame. Not sure on metal frames. You will need the new battery and a short E-TUBE cable, but the wireless app is great as is the ability for the Di2 to talk to my Garmin.