Hi Cameron,
You ask a good question, but I think you need to ask a second question too. Yes, a business directory can generate leads, but the critical success question is: Will any particular business directory help generate the kind of leads you WANT for your particular business?
To answer this question you might want to ask what similar businesses and competitors are doing, and they try to see if it is effective. For example, telephone yellow pages is a type of business directory, but many people find it much more effective to join a fee-based referral group that provides word-of-mouth referrals (e.g. BNI, Chamber of Commerce, etc.) or use social networking (Facebook, Twitter, etc.) to promote their business. This is true with my own business as a business and personal coach; most people want to find me via meeting me, getting to know me via social media, or by being referred by someone who knows me to be an effective coach.
The problem with a directory is that you put your own material in it, and naturlly people expect you to present your best. There are many businesses where people want a referral and not a lead because with a referral you have someone who has witnessed how you do business and can give a firsthand account of your quality and integrity. Do you really want to look in a directory to find your next doctor, lawyer, dentist, coach, financial adviser, etc. when you really want to know what experience others have had, both good and bad?
If your business works better on referrals, rather than leads, then a directory might not be a cost-effective choice. --Hope this helps...