Pros
Great work schedule flexibility in many areas, good pay;noble mission;some great co-workers. If you come from cataloging or librarian background you will fit in to quirky culture. Some of the tech and product areas have good middle managers.
Cons
Inefficient org structures with siloed accountability. Top heavy, arrogant mostly home grown disconnected senior management team. Culture is really old school and completely library focused. Inability to execute on plans and products due to lack of skills and accountability at senior exec level. Middle manager burn out is inordinately high due to irrationality of plans and delivery schedules. If you come from a rational for-profit background you will find the culture maddening for at least the first 5 years.