How strong are the leather straps?

"A question ... your strap is it made from a continuous loop of leather or both ends are tied/hidden within the waxed thread. I need to know if it's the latter cos i will take care not to swing the camera around as the strap ends may just came loose."

There is a glued and stapled joint under the waxed thread. I've really pulled on these with my hands to test them, but I've been thinking about something a little more measurable. VOIGHTLANDER's question prompted me to do a test to see what the limits are. I first hung a vise and hammer that totaled 17 pounds. That worked fine so the only other thing heavier I had was an old monitor. It weighs 25 pounds. It hung there just fine, but the split ring was starting to deform.

At this point I didn't want to add more weight because I didn't want that much falling if it failed.

I think it's safe to say that if the camera you are carrying is starting to deform the split ring that you might not want to swing it around.

I also have been asked "How strong is the string?" on my string attach straps. The string  that attaches to your camera is the material as the wrapping cord. I use a "monkey trap" design: The monkey wants something inside a jar, but when the monkey’s hand fits through the hole his clenched fist can’t fit back out. I use the same concept to keep the attach string from pulling through. 

 The attach string is double knotted and the wrapping cord traps it in place. You can carry a fairly large camera with this arrangement. (Apologies for the slightly blurry pictures. I was using continous lights and a 1.5 second exposure. The camera was hanging free and was moving slightly.)

To test the strength of the attach strings I attached it to my 5x7 Burke & James with it's 10" Elgeet lens. On top is a Zeiss Icon Box Tengor and hanging in front is a Nikon F3 HP with motor drive, a Honeywell Pentax H1a, a Leica IIIc, and a Ricoh Diacord.

The total camera weight is around 20 pounds. I would recommend if you're going to carry all of your cameras with one neck strap, that you also purchase a neck pad. Just a suggestion.