Interesting comments re YP: on our local structural engineers page, it was a case of big dick advertising, with ever-increasing sizing every year between the principal local ones.
lol - That's exactly what my mate found with his engineers practice.
I recall speaking with him about 3 years ago on where he gets his business and he said that about the adverts and he was spending 20k on YP in various editions.
Most of the work was crap from the YP with timewasters and non-payers. I think he reduced the ad and concentrated on repeat clients and recomendation and used some other targeted marketing to generate high-value custom at lower outlay