Unless you're targeting a very old demographic, or are in a very small town / rural area, an ad in the physical Yellow Pages is worthless. 58% of Americans have smartphones at this point, with continual access to Google if not Yelp, FourSquare, etc. And the rest have internet at home.
Any money you would spend on a YellowPages ad is better spent paying someone to link-build, make sure you're in all the local online directories, take some good photos for your Google Places & Yelp profile, etc.
Yellow Pages advertising is more about the physical book (which is pretty much obsolete). Their key value prop nowadays is driving sales to businesses on their web properties (YP.com, etc). So the online properties of yellow pages might indeed be worth the investment depending on your market. But their print advertising is a dying, if not already dead, medium.