Craig's List is a double edged sword. I own a few websites that really benefit from CL, Kijiji, etc. They are information based websites. One covers vintage 4x4's (restorations, vintage photos and event coverage, etc) while the other covers off road racing across Canada. I typically get 100-200 hits per ad and most of them end up reading an average of 2-4 articles. What I am not sure is if I am retaining them and if they are coming back to the site the following week, etc.
However, like others said, I have found that a lot of CL users to be interested in price only. They only want the cheapest and do not want to compare quality to the price. I have posted ads for a few companies and in the end we have decided not to continue. Because the companies deal with high end auto accessories, it was not a good match.
For my racing site, I use google alerts to notify me of upcoming events across Canada. A lot of these alerts come via Craigs List or Kijiji ads and I have made some great contacts. So, if you are planning an event, both venues are a great way to get the word out.