Pros
Great for students as the work schedule can work around your school schedule. No last minute changes or calls telling you to come to work. Generous vacation and holiday policies. You have the ability to earn sick days. Work is straightforward and easy, albeit tiring. You can meet lovely people (staff or patrons), but this really depends on luck and which branch you work at.
Cons
Disorganized management, co-workers with low-morale, not much room for intellectual stimulation if the branch you work at is a busy one. Depending on the people you work with, you're given little to no respect. The work is unsanitary and can be physically taxing (especially at a busy branch, where oftentimes you're expected to run floor to floor in a hurry to get things done). Breaks are short (no time to eat). If you show you're good at your work, prepare to do the work of your co-workers too (not necessarily a bad thing, but the low wage makes this unrewarding). Not everyone knows what they're doing, so you'll be fixing other people's mistakes very often (and they'll keep making the mistakes in most cases, even if you try to address the problem).