Carnegie has a very good collection of art from many different styles. They have an impressive standard exhibit of the old masters (impressionists, cubists, etc etc) as well as some newer modern art.
Their special exhibits are outstanding. Many of the pieces in these exhibits are imported from the Louvre and various museums in NY and London-- not items that you can usually see in person unless you're able to travel extensively.
Good museum, courteous staff, very reasonable admission. Highly recommended.