I have found web hosting directories to be an ineffective way to market for new customers. There are simply too many hosting companies on the Internet competing for the savviest customers (the ones who know where to look). I have found that "networking" with companies and individuals in the "real world" is a much better way to gain contacts and customers and providing a good service for a reasonable price allows one to keep existing customers.
I am sure there are some companies that have benefited from online hosting directories, but I have not seen any quantifiable results for our organization.
AtlantaWebHost.com, a service of Rietta Solutions
Web Site & Development Hosting, Dedicated, Colocation, DNS, Atlanta-area T1 for Voice & Data