Efficacy of YP advertising rather depends on what you actually do. Although I use YP to find suppliers, I think advertising in there is a waste of time for the little guy.
1. If you are, say, a plumber, then what do you advertise under? Plumbers? Central heating specialists? H&V engineers? And if you do plumbing and also fit, say, bathrooms, this adds another batch of headings. So which do you choose?
2. What if you live on the border of two (or more) YP editions? Which one do you choose?
3. Unless you're lucky or have work in hand, you'll have to wait a few months before you see your advert and get any benefit from it.
4. Customers who search YP are ikely to be getting several comparative quotes. Unless you are confident of being better than the rest, then don't bother.
5. If your advertising is a success, then you will have sufficient work to keep you busy, then you'll have to turn away all subsequent enquiries, and people will be put off phoning you again because you're always out, or too busy.
6. When I was looking for a carpet fitter, I tried every carpet fitting entry in our local YP, but every one was too busy, so I used a boke who had "carpet fitting" written on the side of his van - and to whom I've referred several other satisfied customers.
What you ought to do is identify your target market and find out how to get more directly to that market. For example, because I wanted only local work, I spent about £70 on some decent business cards, and £50 on a classified ad in the freebie paper. Over the following few weeks I gave the cards out willy nilly, and left them with all my customers, and anyone I visited to quote. Now, I don't advertise at all, and I'm permanently in work due to referrals.
Your decision should be based on what you do and where you want to do it.