It's pretty safe. Used it on my 40D + EF 24-105 (with hood) + BG-E2N + 580EX (with DFD Pro) and no issues whatsoever. To feel even safer, probably best to check if it still hangs in tight every once in a while (nope, it didn't loosen in my case).
I've tried it on my D90 with 18-105 & 55-200. On occasions, I may have my SB600 attached. So far so good. Our DSLR is dear to us so it's normal for you to be paranoid. I was paranoid too initially but I guess we need to take the first step.
Frankly, my fear is not about the connection becoming loose, it's more about passerby rushing & knocking onto my DLSR...
I have no problem using my DIY 'Black Rapid System'... my 2 most heavy setup is 40D + Battery Grip + EF-S17-55mm f2.8 IS + 580EX MkII or 40D + Battery Grip + EF 70-200mm f4 IS.... of course, the FastenR is fasten on the 70-200mm tripod collar to better balance the camera. :sweat:
Tried that also when I'm on solo photo assignment - 400D with 70-200mm & 40D with 17-55mm + 580EX MkII.... both camera with battery Grip. Since my strip is DIY.... it is slim and without any pocket (like the 1st GEN Black Rapid). It doesn't take up too much space on my chest.
Looks abit silly.... but it will work. I put on the 400D 1st (keeping the camera on the LEFT side) and the 40D 2nd (keeping the camera on the RIGHT side) Just remember to lengthen the strip of the camera on the LEFT, as it will be 'held down' by the 2nd strip, the extra length would enable the LEFT camera to be move to eye level without pulling too much on the 2nd strip. :thumbsup:
For 2 cameras, you need to be alert when squatting down. Not recommended for those using R strap rarely or beginner. For those using them day in day out, not an issue with 2 straps. Usually my left side is shorter as I am a right hander.