In the end, surprisingly enough, this beer turned out rather well.
For 3 gallons of Stout, I added 2# frozen cranberries and 1# frozen black cherries.
The beer is definitely Stout first, getting a nice roast flavor and some chocolate, nice foam and lacing, black as night. But you can pick up the cranberries in the nose and there is a refreshing tartness from the cranberries in the finish. I find the fruit a bit on the subtle side, but everyone who has tried this beer notices it at different levels. No medicinal flavors whatsoever. The cherries seem to be a bit lost, but I'm thinking they are providing a balance to the cranberries that would be missed if brewed without. Not sure on that, but I don't think I'd leave them out if I do this again. I might consider upping the cranberry dose another half pound, turn the dial another click, just to see. Anyway, the moral of the story for me is that frozen, grocery store fruit *can* work just fine.
As an aside, after a week on the fruit, and after racking this beer to the keg, I noticed the condition of the fruit left in the fermenter. The cherries looked completely spent: brown, mushy, nothing to make you think they might have once been cherries. The cranberries looked a little water-logged and maybe not quite as bright red as straight out of the package, but for the most part they looked exactly like cranberries. I did try banging the bags off the counter a bit before adding the fruit, but maybe I could have busted the cranberries up better? Or maybe they are just tough buggers.