Never, ever!
I've done at least a dozen (successful) job searches since the introduction of job boards, and I have never gotten a job from one of them. With that said, posting my resume to these job boards has resulted in successful placements, but never to a job that I applied directly for. It was always the scenario where a recruiter then found my resume on the job board, and placed me to a different opening - most of which were never listed on the job board.