Pros
You learn something new every day, there are lots of opportunities to lead programs in your area of interest, most staff are inspirational at how well they work with a difficult population, and outstanding benefits help make up for a middling (at best) salary. Each branch offers a different experience and you can work with a much different population from one branch to the next. Lots of opportunities to make a difference in someone's life, even a small one.
Cons
Newer staff are thrown in front of the bus every summer only to be pulled back at the last second as part of the library's annual "budget dance" with the city. Many supervisors are painfully resistant to change, and there is a lot of unnecessary bureaucracy in general system-wide. A new "self check-out" model is leading to the blurring of the roles of librarians and clerks at many locations. Constant budget cuts mean inadequate materials, the inability to adapt to rapidly changing needs, and dissatisfied customers. Most of the buildings are dirty, overheated, and generally inadequate. Most disturbingly, due to the recession and budget cuts in social services, the number of people using the library who are very troubled and sometimes potentially dangerous is increasing, and security in the libraries has been reduced, leading many staff to feel unsafe. Even when there is an officer, you're usually on your own to deal with the problems. All that said, working at BPL really is an interesting and often rewarding experience -- but the constant budget problems are really interfering with the library's function and the morale of its employees.