The relatively low effectiveness has been documented, e.g.:
This is in spite of the TSA taking action against anyone reporting the results of effectiveness tests:
Also, there are cases of testing being rigged:
This is just what I can immediately find via Google. I've seen many similar reports, but I don't make a habit of archiving every news story I read for the sake of future usenet discussions.
Beyond that, I wouldn't expect manual examination of X-ray (and similar) images to be particularly effective. Monotonous tasks have a habit of making the person performing them somewhat less than alert.