Pros
Some of the people are nice, the job has quite a bit of variety. As a CSS you assist patrons with self-checkout, paying fines, getting a new card, answering general questions, unloading materials that come from the delivery truck from other libraries and preparing returned items for checking out. As a Sub, you can select your shifts from alternate libraries within the system.
Cons
You are limited to only working a set amount of hours per year. You cannot go over this. It averages out to about 30 per week if you want to work all year. There is a different environment at each library, particularly with staff. As a Sub CSS, you have to follow a different set of rules than the employees assigned to each library. If you see the employees at a library doing something like making a personal call off break during a slow period do not assume that these rules apply to you. It also doesn't matter if you ask permission and receive it, you WILL get in trouble later. Most of the employees are women and, unfortunately, there is a bit of back-biting and patronizing behavior, particularly at certain branches. Employees at branches will ALWAYS have preference over you. If they are rude to you and then turn around and say you were rude to them they will be taken at their word. You will not. It will be a lot of wringing of the hands and "Oh, perhaps you took it the wrong way" when you bring it up with a supervisor. (I'm trying to be objective here.)