I live in NYC and had never made it to see this place, it took a friend from out of town who was visiting to get me to go here and it was amazing. The original part of the library is a part of american history which shows how a very small percentage of Americans lived during the turn of the century in the "Gilded Age"
The new galleries had an interesting assortment of collections for show, one of them being Mathew Barney, whose work I would never think of pairing off with the antiquities in the museum/library - but it was all really interesting.
The gift/book store is worth visiting, and the cafe is nice too (they also have a fancier restaurant but we didn't try that). If you've seen all the major museums in the city - without a doubt, come and check this one out!