Pros
To some degree, you can decide when your SI sessions (tutoring/study sessions) take place. You must, however, attend the lectures of the courses for which you are serving as an SI. The job is pretty easy if you understand the course; you're mainly just explaining concepts from the course to students and helping them work problems, complete assignments, and prep for tests. It's easy to fit in another part-time job with this job. If you have no teaching experience but are considering teaching in the future, this is a way to get some useful experience.
Cons
Many of the people running the SI department are a bit "silly," for lack of a better word. They have a lot of naive ideas about how you should conduct your sessions. Luckily, they mostly just talk about these ideas early in the semester and in their email newsletters and don't interfere with your work as long as you complete your tasks and no students complain.