Shimano Ultegra 6870 DI2 Rear Derailleur 11 Speed
More compact and lightweight, the new re-designed rear derailleur has a wide link design to reduce shifting deflection and now the mid-cage GS version accommodates up to a 32T cog. The E-Tube connection simplifies installation and provides all-weather protection for reliable shifting in trying conditions. The rear derailleur continues automatic crash protection features.
I've been using Di2 for about 2 months. Gotta say that I love it. Super smooth shifting and easy setup. My Ultegra manual groupo was great but this brings it to a new level. Yes, there is a battery involved-- mine is the internal seatpost battery making it less obvious that it's electronic-- but the charge lasts so long it's a non-issue.