How does Di2 work with a 5X?

This is months late, and the flippant answer is "very well". Di2 connects just fine, and if you want data screens displaying whatever data, it can give it to you. I have used the 5x with an Alfine system with an old SM-EWW01 D-Fly unit, an Ultegra system with a WU111 in-line D-Fly/Bluetooth unit, and an XTR system with a SC-MT800 D-Fly/BT display. The cool thing about the 5x is once you get all these sensors programmed in, it just detects them when you choose "Bike" as the activity. Whichever bike you're on, it picks up those sensors. Usually the D-Fly is the last thing to connect since it doesn't wake up until you perform a shift with it. That's fine. You'll see <name> connected on your screen for a few seconds. When you initially add a Di2 system, you'll program the chainrings and cassette equipped. Not sure what Garmin does with this info. This works with everything but the Alfine system, which isn't really supported in this manner.

During rides, it records gear position. This is accessible from the GPX files, but Garmin Connect doesn't really do anything with it. There are other websites you can upload your ride files to and get metrics out of them. The watch DOES record how many shifts you performed on a ride, and it will BLOW YOUR MIND to see how much you shift. Roughly 1100 shifts per hour while riding a touring bike for me. You CAN access all the fields you want through data screens, though I don't do this. What i've used D-Fly for on other GPSes, ones that display to a screen I can glance at casually, is a visual display of the chain position on the rings and cassette. I can see when the synchro shift is about to drop me to the small ring. Otherwise, i'll check the battery level every few months. This isn't really a use case for me, since my road bike is the only one that doesn't have an OLED display in the system.