I thought the Di2 is a cool idea.
I read about it in the MTB Magazine I picked up for my Rome trip.
They had it all mounted to a Mtn bike and said it worked good. The front auto trims, and the rear can auto adjust and align.
I think with some more development Shimano might have something here. They battery also lasts quite some time.(3 months? if I remember correctly)
I like mechanical things, and usually dislike computers running everything, but I like the concept of Di2.
The magazine said that it provided very smooth and responsive shifting.