West Marin County especially almost had no reception at all. This was surprising to me because of the population density and the amount of tourism and residents. I would guess it is also a safety nightmare for emergency services in the area. There are long stretches of road that have no signal and the roads are dangerous. There are bicyclist all over the narrow roads that don't have any shoulder to ride on. The roads are steep, have lots of blind curves and make for hazardous driving conditions if you are not alert behind the wheel. I would imagine there are lot of car accidents.
Here is a list of cities, highways and locations in the area that had NO RECEPTION for T-Mobile and Verizon. AT&T had a weak signal in many of these locations.
Bolinas - No signal
San Anselmo - No signal
Muir Woods National Monument - No Signal
Highway 1 in West Marin - No Signal most of the way
Panoramoic Highway - No Signal
Ridgecrest Blvd. - No Signal
Sleepy Hollow - No Signal
Butterfield Rd. - No Signal
Stinson Beach - 2 bars of 2G Edge on T-Mobile
AT&T probably had the best reception in the area and this seemed to be the carrier of choice for most residents in the area. If any reception was available it was likely 2G and downloading data or email was virtually impossible. I would imagine that there are lots of AT&T Microcells used by residents in the area.
I was not able to review Sprint's coverage in the area but it is likely not much better. Thankfully I am a T-Mobile customer and was able to make and receive phone calls through WiFi at our friends house. T-Mobile WiFi phone calls save the day once again.
I would imagine there is lot of resistance from the residents for installing cell phone towers in the area. I would characterize the population of residents to be quite "earthy". I have found these communities and the City Council's o be quite resistant to the installation of cell phone towers or new antennas.