The 2 main reasons for being listed in web a directory is increased exposure and back link building. When looking for directories to submit a site to I would consider directories that had good Domain Authority and Page Rank.
Keep in mind that most reputable directories are usually human edited, have not been spammed to death and thus harder to get accepted into. Many directories just dont add any SEO benefits but one directory that is worth submitting to is dmoz.org.
Before submitting a site I would make sure that the domain was at least 2 months old and had some quality content on it and then submit your site to the category in the directory related to your niche and wait for approval which can take anywhere from days to months depending on the directory.
I just had the same question some weeks ago. I found a lot of directories via Google, but a lot of them were very old and not updated for ages. So as in addition to said above I would recommend you to pay attention to this point.
It could work perfectly . But you would need a plan to get ahead of the competition. One of the thing would be reaching both clients and people to contact your clients using your directory . This takes a lot of time . After you have proofs that your directory work - you're on the right track to build a succesful bussiness out of it.