I've been riding with SRAM Red eTap for a year now and am sold on electronic shifting. I chose eTap over Di2 for its cleaner look; that is, no electronic cabling and no junction box / battery under the stem.
I do like the shifting performance, particularly on the cassette. I find that I shift more and, as a result, tend to maintain a more constant power output. And, when coming to a stoplight / sign, shifting to the largest cassette gear is easy, simply hold the shifter lever for multiple shifts.
Keeping the batteries charged is not a hassle. Typically, after a ride, placing the bike on my work stand, I shift rear, then front, watching the LEDs on the derailleurs and shifting levers. Then, charge if other than green.
However, here is my gripe. The mounting tabs on both derailleur batteries have broken off. For a picture, go here
https://www.amazon.com/SRAM-eTap-Bat...dp/B0186OPWJA/
and look at the reviews. One of the gents uploaded three images.
Contacting SRAM is uesless. The batteries are EXPENSIVE!
I have resorted to securing the batteries with a zip-tie loop.
Oh well.