Crystal is one happy boxer!
I wouldn't leave an untested dog alone with a marrow bone because there ARE dogs that will not stop chewing/gnawing, and unlimited access can really damage teeth.
Mine go at them like crazy when they're all new and gucky, but I limit it to about an hour or so for the first couple of days. Once they're dry, they seldom do more than give a few gnaws and then use it as a chin rest.
Once dry, they also make good alternative kongs -- you can stuff banana into one for cheap entertainment (I don't use a whole banana, just enough to fill the middle without easy access at either end. Gets those tongues working!)
And I don't leave dogs loose with them until they're trained enough to stay on their own rug with their own bone. And again, that's under supervision.