I got my 6850 Di2 11 speed bike in Sept 2014. Approx 35,000 miles and 9 years later, it still works perfectly. I replaced the seatpost battery about 4 years ago when it started needing recharges once a week. I estimate I've done between 500,000 and 700,000 shifts on this bike. No maintenance other than lubing the pivots on the derailleurs occasionally. And a rear derailleur fine tuning maybe once a year -- the shifting is "slightly noisy" so I hold the junction box button for 5 seconds to get in micro adjust mode, click the up or down button once or twice to move the shift point slightly in that direction. Fast and easy.
I did replace the rear derailleur as a preventative maintenance thing, since I plan to ride this for many more years and worried about spare parts far in the future. The 8100 rear derailleur works with the rest of the 6850 components.
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I can shift with my ring finger if needed. Shift the front without needing finger leverage to finish the shift. Shift for a few pedal strokes, then shift back, without really thinking about it. (I like having the "just right" cadence.)
The shift buttons even work fine with thick ski gloves in winter weather.