I use this on a Cannondale SuperSix High Mod that was originally equipped with a Dura Ace 9000 rear derailleur. While the DA 9000 shifting was excellent, it was a short-cage derailleur, and I wanted to switch to an 11-30 cassette. I am now 71 years of age and I still regularly climb gradients that can easily exceed 10%. The Dura Ace R9100 coupled with an Ultegra 11-30 cassette has not only made for quieter shifting, but it has made it just a little bit easier to climb the most challenging hills around my neighborhood. I also like the shadow design, and, even though I purchased B-Stock at a nice discount, there were no flaws in the derailleur whatsoever. I am very happy with this purchase.
Precise shifting. Light. Easy to install. Only issue I have had so far is that the pulley/jockey wheels are on the noisy side of things. Nothing alarming, but it seems to be taking more time to "break in" than Ultegra.
I'm an older, semi-fast recreational road rider with over 30 years experience with top of the line group sets. These include Campy, Shimano and even Suntour. I had a full Dura Ace 7900 group on my Litespeed Archon T1 and the shifting was dialed. In fact everything worked so damn well. But then I decided to do an upgrade to a Dura Ace 9100 mechanical group, mostly because I just dig the hell out of the way it looks. After I installed, adjusted and rode the new set up, I was just amazed that the front and rear shifting was far better than the 7900 group. Another thing that I'll have to wait to verify, but the drivetrain is silent. I haven't had much riding time on it, maybe 150 miles and haven't lubed the Dura Ace chain yet. It still has the packing grease. I'll see what it sounds like when I lube it.
The reason I only gave 4 stars for value is that I'm pretty sure Ultegra would function just as well, but at 12 the price. The glossy finish on the Dura Ace makes me swoon.