I met my BF on Craigslist. Seriously. We've been together almost three years and while I'm getting ready to break-up with him, we *have* built a multi-million dollar business together so it isn't all that bad. And when I am single, I'll probably be using CL to look for a new apartment and possibly new dating possibilities.
I've also bought and sold things on the site and I see no reason why you shouldn't at least APPLY for a job you found on that site. It isn't like you are committing to accepting the job and if you don't even investigate, than you might be missing out on a great opportunity. So what if a company doesn't list who they are? A lot don't. I say go for it.