The d-fly unit is purely a plaything. It adds no speed, power or aero benefits. The information it gives is like the froth on a cappuccino - nice but of no use whatsoever. Set up is not straight forward. It should be plug and play but unless you have done one before, I’m sure you’ll have to rely on YouTube to set it up. Even then I had to go through the process 2 or 3 times As others have said - it should come as a standard part of the Di2 kit but Shimanos accountants have no doubt seen the opportunity for another profit centre. You’ll need what may be the worlds most expensive cable - £20 or so for a 6” lv connector which probably cost them a matter of pennies to manufacture.

Great addition to fully utilise my Ultegra Di2 groupset. Allows me to integrate my bike with my head unit and also not worry about the amount of battery I have left. The only downside is that I am not sure why it isn’t included when you buy a bike in the first place