This device enable wireless feature on Di2 system, Di2 is wired FYI. With this device, system can communicate wirelessly for update firmware, customized button at the shifter, Garmin or alike devices. Almost plug n play, well just need to update firmware...you good to go.
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.
It took some work to get everything set up (shoutout to my mechanic The Rover), but once he made the battery upgrade, updating the firmware was a snap.
Honestly, I was considering experimenting with the newer possible settings afforded the Di2 user for more intuitive shifting, but the instructions on the e-Tube project app are so opaque I don't even want to try it. So it's helpful for keeping the firmware up to date, though this doesn't really change my day to day riding experience, so it's a bit of an unnecessary toy!
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.