Not only a Pittsburgh landmark, but one of the most unique and original academic buildings in the world, and a national treasure, on the national register of historic places.
I took Slavic language courses here and every day would study in the main hall and walk wander through the various nationality rooms. The view at the top is spectacular and during weekdays there is a decent cafe on the basement floor.
For some trivia, this is the world's second tallest academic building in current use, second to the Moscow State University wedding-cake tiered building. The only reason Moscow State is taller is because it was constructed shortly after by the Communist government and they put an unoccupied metal pole to claim the title.
I've been in both buildings, and the Cathedral of Learning is far cooler.
Here's some more trivia (or mythology): the cathedral was the inspiration for the final scene in the 1989 movie Batman with Jack Nicholson and Michael Keaton. The one you see in the movie is a mockup, but according to Hollywood lore, Keaton, a Pittsburgh native suggested the cathedral as a model and the final scene. It's also featured in the movie Wonder Boys with Michael Douglas.