I think it depends dramatically on your industry and the policies of the directory in question. Some directories can help your google ranking, but to the most part they seem to be list farms for telemarketing companies IMHO.
Also, it seems easier to get positioning in other forms of social media (especially if you sell overseas) and to use that as your directory if you build up a good enough following. Regular youtube Vlogs are a good thing to try. Some people actually get addicted to watching online adverts anyway and they can become the client base of your future.
Yell is good for plumbers, window fitters and plasterers, some trade specific directories can help with awareness sometimes, but I loathe even more advertising sales call then I am getting already so I try and stick to internet social networking.
However, I don't think that there is a singular black and white answer even though we all want one