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I've been using a Black Rapid strap for over a year, been very happy with it. I've used it with 1D and 5D cameras with various lenses and haven't ever had any issues at all. The metal clip screws shut (simliar to a small locking carabiner), so it's not going to come open by accident, it even screws "down" towards the load, so it won't work it's way loose.
As far as the metal stud that screws into the bottom of the camera, I've used a small paint pen (nail polish works well too ) to make an index mark so I can confirm it's tight by glancing at it.