I think it depends where you are. My site, when it was up, had a #1 to #6 listing on Google and Yahoo with a search for, and any variation of, "window cleaners birmingham" (even without a UK specific search) for nearly 4 years. I picked up about 3 houses!
However, some on here do better, so, like I said, it must depend what area you're in. At the very least, having a web address on vans and stationary looks the business so go for it.