I just rode nine weeks across the Alps with my SRAM X0 Transmission and the battery lasted the whole trip. A spare doesn’t way too much though. A rock hit my transmission hard and it needed slight adjustment. I took less than a second and I did not have to get of my bike. Also, installing the transmission was super easy. All in all, I think it is way more convenient than mechanical shifting, if you are willing to pay the price. It’s probably not worth it for most people.