Shimano has once again improved on a their Dura Ace derailleur when I thought it was impossible to make it any better. This design requires almost no effort to shift, the cable tension adjuster has now been incorporated into the derailleur itself and the cable lock nut and limit screws both allen head screws for ease of adjustment. Plus it looks awesome.
I decided to upgrade my SL3 from Ultegra to Dura Ace, "cool jewelry." The 9100 is self-adjusting, fast and smooth, not to mention very efficient. Di2 on my SL6 S-Works is jealous of this gorgeous mechanical groupset.