Just got a new GPSMAP66st (my old 60CSx died). On paper, it is an impressive improvement over the 60: 16G memory vs 8g; 20,000 track points vs 10,000; 12,000 map tiles vs 4000; GPS Elevation vs barometric altimeter. It seemed like a good idea at the time...

What the user manual doesn't tell you is what it no longer will do:
You cannot display multiple waypoints on the map.Will only display one waypoint at a time on the map. If you select a new waypoint to show on the map, the previous one disappears.
Will not display the current track you are recording while you are recording it. So, you can't see where you've been until you stop recording and save the track. If you are traveling backcountry off trail, this is a real problem.
You cannot reset the Odometer field in the Trip Computer, and you cannot choose individual fields to reset/clear. The only way to reset the Odometer field is to do a global reset that clears all the settings you have entered. Why have a data field that accumulates data and cannot be reset?

I have contacted Garmin Product Support, and they have confirmed that this is the way the 66 is designed, it was not operator error. I don't know what Garmin was thinking when they designed the software for the 66, but they have eliminated a lot of the previous models' functionalities. This is not product evolution, it is product devolution.

I have returned my 66st. I am very disappointed with Garmin.