The Cathedral of Learning is perfect. Maybe I am a little biased because I went to Pitt for my undergraduate degree, am currently enrolled as a graduate student for Pitt as well and have lived in Pittsburgh all of my life... But regardless, I love it.
The building is gorgeous and is often referred to as Hogwarts- for Pitt students anyway :). The nationality rooms are awesome and so unique! I love taking a look in them and checking out the art and all of the work in general, that has been put into them! There's a secret attic in the American room and I was lucky enough to explore it when I was taking one of my classes as an undergrad.
The Cathedral is perfect to study in. Tons of spaces with quiet areas as well as areas for groups to meet. The view from the top is incredible as well!
The Cathedral of Learning is a Pittsburgh landmark and is the heart and soul of The University of Pittsburgh.