I have been doing them for 6 months. My opinion is that they will very little to drive business to you. However if you are doing other things to attract business a billboard will improve your brand awareness and make the other stuff more affective.
An example would be if you do google adwords. When someone looks on the internet for a cleaning service it does make it hard for them to trust a company on the www. So by having my billboards up the customers have been saying stuff like, I have been seeing your trucks.
Having three trucks in a city of 100k means that they probably don't see my trucks that often but seeing the billboard with my van on it does sink in after a while.
So in short, you need to start with the low hanging fruit, direct mail and other stuff. Then as your budget allows the next step is brand awareness by billboards, tv, and radio