The berries can be stored and used throughout the winter--a good way to get vitamin C! This article has a nice description of sumac and sumac-ade. Here are some ways the berries have been used medicinally:
"An infusion of the fruits has been used as a tonic to improve the appetite and as a treatment for diarrhea. The berries are astringent and blood purifier. They were chewed as a remedy for bed-wetting. A tea made from the berries has been used to treat sore throats." (Littleflower)
You can easily tell the edible sumacs (including staghorn and smooth) from the poison sumac, which has white berries. If you are allergic to cashews and mangos, you probably should stay away from all sumacs as they are in the same family.
I want to try using the foraged sumac berries to make the reddish purple powder that is used to garnish hummus and other middle eastern foods. I don't know how to go about separating the seeds from the powder though... Has anyone out there tried this?