US Airways is the dominant carrier at PIT. It operates 504 daily flights and is the airline's biggest hub. The airport is wonderful. It's clean, efficient, has a nice mall, and is new. The midfield X terminal opened in 1992, replacing the old and decrepit Greater Pittsburgh International Airport terminal. The terminal is spotless, clearly marked, and a very pleasant transit point. When I do fly US Airways (in the past and this year, after a five year hiatus, I always try to book connections through PIT if possible and logical).
United, TWA, AA, NW, Continental, Delta, the newest incarnation of Pan Am, and other airlines operate from PIT, but it is overwhelmingly US Airways/Express territory.