I have an RS-4, which might be the most important accessory that I own. I agree with Richard that the Black Rapid straps aren't ideal for many sports, but they are amazing for hiking, trekking through cities, and generally shlepping a camera around for hours on end.

The strap works fine with a backpack. I actually prefer to put the strap on first and then the backpack. I've done exactly that for a 12 day trip in Israel and had no complaints. Having the use of both hands is key, and you don't have anything bulky on your chest.