I used the d-fly unit paired with a Garmin Edge 520 for a couple years. It worked - pretty much. No lockups or interruptions to shifting, but sometimes the 520 and the d-fly would drop connection. In addition to losing the functionality, it would go through reconnecting which was really annoying. I have since upgraded to an Edge 530 and everything has been rock solid.
To me, the big advantages are control of screens with the top buttons, seeing the actual battery level of the Di2, and seeing the gears that I'm in. I wish Shimano would have built in the connectivity, but at $75 plus the cost of a cable, it's worth it to me.