Took this route yesterday. Wow...... It was a Sunday and 50% of the traffic were bikers.
Highway 121 is known locally as the Monticello Road. This is a real riders road. Leaving Napa, it climbs the hills towards Lake Berryessa. Smooth pavement and never-ending curves make 121 a real thrill ride. It ends at the junction with Highway 128. From there, you can turn east towards the town of Winters, or west back towards the Napa Valley.