Bejing is in a Penn Institution. Located right near the Quad, one of the central dormitories on campus, it is always packed with students and professors alike. A favorite for its quickness, it is also VERY fresh and VERY good, basic Chinese.
Don't expect to sit around for an hour after lunch- they certainly move the people in and out in a hurry. But it is well worth the lack of ambiance for the great food.