Yeah, I like the flop test myself but I also agree that depending on the tightness of the cryovac, I can't always get a good reading. Seems, however, that even if it doesn't "flop" well, but still feels soft, then once it's unwrapped it'll fold nicely like landarc's photo above.
I read Mr. Biggles' comment some time ago that if you start out with a set of muscles that is already limber, soft and has a yielding texture, then the finish product is likely to be better (providing you don't fark it up during the cook) than a brisket that starts out stiff and rigid.
Face it....the briskets that come out of the large slaughterhouses are from all SORTS of cows (size, age, feed, etc). What OTHER than the flop test and general appearance do we have to go off of in the store?
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team