Craigslist is a supremely popular listings site. Craigslist was founded by Craig Newmark in March, 1995, as an email list site for San Francisco and Bay Area events.
In June 2000, it added its second city, Boston, and then expanded to the major metropolitan cities in August 2000. As of February 2008, Craigslist now covers 450 in 50 US States and 50 Countries. Craiglist.org – note the .org domain suffix – is interesting in that it serves no ads and stresses a non-corporate culture. It has only 25 employees, all based in San Francisco, and makes money off job ads in 10 cities, as well as brokered apartment listings in NYC.
Craigslist (CL) national reach in terms of listings completeness is very difficult to ascertain. It's better to think of CL from a regional perspective. Some regions, like the Bay Area and New York City, have very active communities posting real estate and apartment listings to CL. Other regions have much smaller communities and thus have fewer real estate listings. Your results may vary.
Craigslist (CL) is functional and easy-to-use but not fancy. Very little has been done to modernize the interface since its founding and many CL users like that just fine. Our feeling is the somewhat archaic search and sorting tools could use an update.
Overall listing quality on Craigslist (CL) is quite good. However, because CL receives a deluge of listings on a daily, hourly basis, there tends to be quite a bit of "noise" within the listings -- an annoying percentage of spammers and promoters. It can take some patience to sort through it all. Our testing also indicates a relatively high number of real estate agents posting "bait-n-switch" expired listings - a way to get you to call them and give them a lead. You can find some excellent listings -- it can just takes some work.