I have Di-2 on mine, and it is fantastic, can't imagine I can ever go back now.

When I was looking at new bikes, I tested quite a number of them, and when I tested a bike with and without the Di-2 within a few minutes of each other, and with almost all else being equal, I was hooked.

It shifts flawlessly, front shifts are just as easy as back. Front self-adjusts with moves through the back. Battery life is not a problem, I have had my bike for just over 6 months, have ridden over 3000km, and charged it just once, and that was about 6 weeks ago. As for battery positioning, mine is under my bottom bracket.

If anyone is keen to see how I have mine set up, I can put up some pics a bit later.