There are actually 4 museums in the Carnegie Museum group in Pittsburgh. I'll review the Natural History and Art Museums in this review (separate review for the Warhol forthcoming.)
The low-down - for $15 you get admission to both the Natural History Museum and the Art Museum. It is well worth the money as you can spend hours on both sides.
The Natural History Museum is a dinosaur lover heaven. The fossils are seriously awesome, especially the one that you see when you walk in (that is still in the huge rock face.) Just a great collection and definitely a good place for families.
The art museum has some great items in it - the Monet probably stands out for most people. I loved the "Chariot of Aurora" installation - a 32 panel wall that once graced the luxury cruiseliner Normandie. There were plenty of gems in the modern collection besides the Monet - loved the John Singer Sargent and the Bourgerou.
Also impressive was the Hall of Architecture and the Hall of Sculpture. These transform you into another place and time while you are there.
I thoroughly enjoyed the time I spent here and would not hesitate to come here again. Highly recommended.