Wired analyzes why craigslist sucks so bad. The article makes clear it is run by a bunch of aspergers types that don't understand how to run a business or design a site.
Interesting insight there, the reason that they don't make these changes that experts suggest is "It is the same reason that craigslist has never done any of the things that would win approval among Web entrepreneurs, which is the same reason he has never updated its 1999-era Web design. The reason is that craigslist's users are not asking for such changes."
I looked at some of their recommended changes in a sidebar and didn't see the advantage, it was like criticisms that google should run more ads on the front page, and have news stories and comics there too like yahoo.
To check this out, I just went and checked CL for local jobs, which I have bookmarked. So there is a page right there that has all the jobs in my city, listed, and one click shows the ad.
Then I went to Joel's board and there was a list of jobs in India and London and places I don't care about. Then went to check out Monster and couldn't find the right part of the site within 10 minutes, and the whole thing was too slow to use.
So, I say craigslist wins not because it is free for most listings, but because it has the information I am looking for and is not a bloated piece of shit like 100% of the other web crapware out there.