This is a must-see if you are visiting Pittsburgh. The architecture inside is amazing, and in the winter time they even light the huge fireplaces on the first floor. Give yourself some time to check out all of the Nationality Rooms as well. Pitt has a wireless network in the building, so a lot of students hide away and study there (or just procrastinate by web surfing).
Secret tip - take the elevators to the 35th/36th floor - the view is fantastic!