This works well - until it falls off. Unfortunately, the "C" style clamp that fits on the runner of the straps is only a strong as the tensile strength of a plastic "open-C" clamp can be. I'm buying a replacement as mine just fell off someplace and is gone. While I had it, it had come off several times - not frequently, but often enough - both while wearing it, and also when it got lodged in something when the pack was off, coming off when you picked the pack up.
The release mechanism works well - one -anded, which I like. It's the "C" clamp that needs rework. Since that is unlikely, my advice - and what I'll do when I get the new one - is to find the position across your sternum that works best, then put a few stitches of dental floss to hold it in place. You can always cut the floss loose if you absolutely have to change the position (say, if someone else of a vastly different height is going to use it), but that will prevent the strap from disappearing on you. You might have to put a stitch on both end, as the use the same "C" clamp to slide over the runner.