"There was a thread here (can't find the link) about someone hanging food up for a month in their back yard and the opsack was never touched (vs a control)."
That may have been the one from me. But great care was used in the kitchen at home to get the food into the Opsac (and the Ziplocks in the control bag) without getting any traces on the outside. The purpose of the test was to see if the Opsacs would work for caches hung from limbs in Ursacks. Since then, caches hung in this manner, but much higher, for over a week below tree line in Colorado were not disturbed. Both were not far from areas where backpackers camp, one in the Rawahs, and one on the CDT just off Rainbow Road. People were just as big a concern as bears and other critters, so the caches were well concealed.
But the Opsacs were used only once, double bagged one Opsac over another, and the outer one secured with the 'Clip 'n Seal' bag clips available from Amazon. All that went into a no longer sold green Ursack that was prepared at home, and hung high.
For daily use, it is doubtful that one could avoid getting food odors on the exterior of the Opsacs. I just hang food as high as I can throw a rock in a Kevlar Ursack that is intended for smaller critters, and have never had a problem in the lower Rockies or the Northeastern US, except for the one time I foolishly left a bottle of blueberry kool-aid next to the tent near the AT. Since I seldom camp above timberline in Colorado in order to avoid lightning exposure, there is no issue about finding a tree.
Away from campgrounds, the bears don't seem to be much of a problem in Colorado or the NE, but in other places frequented by bears used to people with food, it seems from the many threads on BPL that canisters work, except in the instances when bears make off with them. Maybe they can be secured somehow against this.
My only foray into serious bear country involved grizzes in the Canadian Rockies, and they went through every inch of my pack even with no food in it - just the odors transferred into the pack through plastic food bags and a stuff sack may have brought them lumbering into the campsite. Since solitude is a major reason I backpack, I avoid such areas now. Good luck on those of you who don't.