I just moved some of the Di2 parts from my pure race bike to my audax / winter trainer bike, and now I think that I'll leave them there for good and buy the replacement parts for the race bike next spring. My setup is now quite unique - a drop-bar with a right-hand Ultegra Di2 shifter (plus satellite shifters on the bar top and aerobar - multiple shift buttons/locations is one of the main advantages of Di2 for me) and an Ultegra mechanical left-hand shifter (because I already have flawless front shifting, so why spend money on the extra Di2 parts that I don't need?).
It's true that you can get good shifting on the rear derailleur with mechanical shifters and derailleurs, but Di2 takes it to a whole other level of crispness and ease of use - don't knock it until you've tried it. However, once you've got used to electronic shifting then you may never want to go back - which was what happened to me. Battery life isn't an issue because you get 1000+ miles (1500+ km) on one charge, and with the internal battery you have a port to recharge the battery from a USB socket, so I can charge it via my dynamo hub and B+M Luxos U light.
Last edited by Chris_W; 09-30-14 at 03:03 PM.