I posted a employer ad on SO Careers, and also GitHub Jobs. The SO post resulted in a satisfyingly large set of high-quality candidates. I was satisfied with the value.

In contrast, GitHub was surprisingly disappointing. Maybe one or two worthwhile candidates, and a ton of junk. It appears that GitHub's jobs get scraped and posted elsewhere, and the bulk of responses to my ad were low-quality (and that's being generous) candidates responding to those reposted versions.