The only problem I've ever had with my Garmin 800 is that the battery eventually (6 years) wore out, i.e. stopped holding a charge for very long. I sent it back to Garmin with $60.00 for a new battery and all is good now.
I rode a double century last Saturday and didn't even have to re-charge in route like I usually do. Still, I know batteries have life-spans and probably always will. I can deal with that.
However, I did have a problem with Garmin Connect for a few days. Every time I added a ride description it took my total mileage for the ride to zero. After an update, however, all is good again.
Rick / OCRR