Why are you and other people saying at its current state its a terrible idea for DHIve spent a good amount of time working with di2 on road and tt setups and I can say in its current state, a terrible idea for downhill. It's awesome for road bikes and by far the best option for an aero bar shifter, but it would need a re-design for dh or mtb. I think the technology is legitimate and with some changes could work well for dh.
For one, there is less spring tension on the cage than regular DA (probably less work for the battery I would imagine)
You can also only shift one gear at a time (one button click=one shift, both directions) If you want to downshift many gears at once you have to mash the button over and over.
The parallelogram part does not swing freely, you cannot grab the derailleur and move it through gears with your hand. If you force the derailleur to move like this, it will break. This means that if you hit it, the internals will break even if the derailleur itself remains attached and intact.
Last, and most importantly, the derailleur alone is 900 dollars, and if you cut or damage the electric cable, a new wiring harness is nearly 300 dollars
that being said when the price comes way down, and maybe is redesigned for mountain biking, I could see it being pretty good.
Of course it is at the moment!! its for road use... Its not designed for DH!!!
Wait For it to be designed for our specific sport.. No point in putting di2 down as its not for us yet