Carterwrote a review of Shimano XTR Di2 RD-M9050 Rear Derailleur on January 16, 2016
Familiarity: I've used it once or twice and have initial impressions
Have Dura Ace Di2 11-speed on the road steed, now rolling with it on my mt steed :) Total FYI - I am running a SRAM 10-42 cassette, SRAM chain, and SRAM Xo1 1X cranks with this - works flawlessly! I had many hesitations because I love the 1X drivetrain SRAM has to offer as well as the hill grinding 10-42 cassette. Setting up is waaaay easier then running cables, etc.
Only hard part is, that depending on your frame, where to put the battery and wires. Pre-planning wiring and battery placement is a must prior to install. I have a 2015 Stumpjumper FSR 650b and was able to locate the battery in in the seat tube, just below the dropper post. For the short runs of wires that are exposed, I used electrical shrink-wrap tubing for a super clean look.
Long term test to follow. I am familiar with the E-Tube set-up and set the rear derailleur to 'fast' - awesome. Also, if you run Di2 on your road bike, the front LED Display already has a junction box, so no need for that. Additionally - if you get the new FOX shocks, you can remotely actuate the shock setting with this stuff also - super crazy.