I lost my Zumo 660 yesterday at about 50 MPH.
I was using the bar mount that comes with the 660. The mount lock gave up for some reason or another and the 660 disapeared in my left mirror. I was on a back road with no other traffic so I pulled a U and went back to pick of the pieces. 9 pieces in all mostly little pieces of plastic.
I wondered about the mount when I say it for the first time, I wish now that it had a security lock like the 550. I don't know how the locking mechanism released by itself but it did.
I had the 660 on the bike for over 6 months now and it only comes off when I wash the bike or leave my bike when I am out and about. It has not been off for months because of the bad winter hear in Delaware so I know it was seated and locked fully.
I did a search on Google and did not find any one else who had this problem so I am thinking I just had a bad mount or bad juju. Luckily I have both the 550 and 660 on my bike so I am not out a GPS, but I am going to think about buying another 660 for awhile.