I've decided to keep mine restricted to the (movable) run. I have only 3 chickens, but my yard is small and there are neighbors on all sides. I worry my hens will wander into neighbors yard, which is behind a screen of tall grasses, and then I won't be able to see them or get them to come back to our yard. I don't think my neighbors will appreciate them pooping and digging up their gardens.
Also, we have many raccoons and skunks in the neighborhood; and sometimes hawks. I figure mine 'free range' within the safe confines of the run, which we move around as needed so they always have fresh 'range' to graze and explore.