I think Carnegie Library on Forbes is a great freaking library. With that said, it is lacking in many areas. Alot of times they only have one or two copies of very recent, very popular books. In a city of 1,000,000+ people this doesn't work very well.
The renovation was amazing-- the new computer and DVD sections are amazing.
If you're looking for more scholastic books, you can always go to the Hillman library if you goto Pitt (or the CMU libraries). This is a library for the masses, not for the researcher.