Yup, done it on a Gen 2 Speed Concept, upgraded the entire thing from mech to Di2 and basically rebuilt the entire bike.
It is not a nightmare assuming you do your homework. Make sure you read up on how to do it, measure your cable runs, make sure you get the correct battery (one that can do synchro), the correct A junction (I put mine in the plug in and made it very sexy), get your B junctions (1 or 2 depending on your A box configuration) and if you do it piece by piece you can save money but understand there is no “easy” manual for it. You can also buy it all on ebay and spend maybe an extra hundred or 2 to limit headache; almost suggest this is better. Also think about your battery placement, I suggest in the seatpost if it can fit, but wrap the battery in foam and stuff it in. Also I suggest the wireless controller.
You need the Di2 plug/unplugging tool. Also suggest looking at how to get into the BB area and potentially get a tool if you need it to be able to work in the BB area. Also some guide wires may help, you can macgyver some old shifter cables to help. You also need the charger and the cord. You absolutely need that.
It is worth it if you have the funds. Does it make you faster?
Naw.
Is it nicer in every way and make sexy sounds and is just satisfying in general?
Hell.
Yes.