I posted something on craigslist and got 10 or so email responses and a couple of phone calls. It was a good experience and I definately got calls about my ad. I think it depends on the quality of the ad you post thought. Good luck.
Craigslist is huge out in the San Fran bay area… it has expanded to other markets now… response will be I suspect proportional to the number of people who know and use it in your city… different cities have differing levels of readership/usership.
As Craigslist’s only marketing to my knowlege is word of mouth.