Prince George's County is known to have a higher than average crime-rate for the metro area. However, as in most places, most of the worst crime occurs between people who are already acquainted, or are associated with drugs.
In addition, Metro parking lots in Prince George's County, especially Greenbelt and New Carollton, have higher than average incidences of car break-ins; avoid leaving anything of value in sight when leaving your car in these parking lots.