What's odd, is that we DON'T get asked much about them here. I would say less than 5% of homes and businesses that we work on have a rain sensor. And it's probably closer to 1-2%. Even with rebates from some of the water districts.
I also find sensors are a great source of screw ups that can damage a lawn in the heat of summer when they fail in the open circuit position - squirrels chewing wires, wire falls out in the terminal ports, someone did a field wired sensor and the connections came apart. In a climate that sees 16" a year of moisture, every little bit helps. I agree with having them not rain during a storm, but if it rains on Wednesday and one of my TWO days to water in a week is Friday, I want it to still water that Friday.