Radio is VERY region dependent. I don't use it for this business but I used it extensively for my retail store and it was one of the top advertising values for us. We were able to buy up large packages at the end of the year so we were actually running around 5-6k spots a year. Running a small number of ads was very expensive, large quantities were much cheaper.
I did pretty extensive lead tracking with that company and feel confident that the leads were properly attributed.
Finding the right approach for your area is also critical. I ran fairly generic ads prepared by professionals for a couple years but found that people responded better to the less professional, more humorous, ads that I recorded with the station.