Directories are not really great traffic sources unless the directory is very niche related or local relevance. At least this is what I've observed. I do think that some directories provide SEO value but it seems that its more for local marketing these days. If you are using directories as your main form for backlinking, you are wasting your time. There are a few decent directory links still available as previously noted but I wouldn't strain yourself trying to get a link from these sources.
Some solid local and niche directories can provide a place for you to get a valuable citation for your local marketing campaign. In the local market, its about Name, Address and Phone number. You won't do a bang up job with traffic but it will serve to support your local SEO efforts. I don't think directories are for mainstream use any longer.
As for paid listings, I've often wondered and asked many times around here but never saw anything compelling advising to buy a directory links. Your mileage may vary though. I'm sure you can take the money you would invest for a paid directory and invest it elsewhere and get more value from it, such as content creation.
"Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime." Chinese Proverb