Hap Seligawrote a review of Shimano Ultegra Di2 RD-6870 Rear Derailleur on June 25, 2016
Familiarity: I've put it through the wringer
I've been extremely impressed with the performance of the Ultegra Di2 rear derailleur. I've logged considerable miles off road on my gravel bike, and while it's been banged around a bit, it hasn't missed a beat. It still delivers precise, quick shifts every time.