Like the size, battery life, and menu selections, especially the ride history. However for the senior rider who needs reading glasses but doesn't like riding with them, the smaller display can be difficult to read.
This was a huge improvement over the original Garmin Cadence Sensor. Garmin got rid of the crank arm mounted magnet and the chain stay mounted sensor. It now comes in one neat unit that easily mounts on the crank arm. The worst thing i could say about this unit is that my Garmin Edge 800 takes a few seconds to find the cadence sensor.
I have been using this for training and racing. The course mapping is improved over the 520. No problem finding heart rate monitors, smart trainers or power meters. Multiple displays can be set up for each type of riding that you are doing. You can have your main screen focus on heart rate, power, cadence and time and then have other screens to check lap data and so on. You can have turn-by-turn info for your course too. So far, I have used it in a race, for training and on a road bike (Favero Assioma power) and mountain bike (Stages power). Western Bike Works had a really good price for the unit.