Pros
Most of the teachers and other staff members with whom I am working are friendly and respectful to me, and ESS also seems to genuinely care about its employees. The application and hiring process is also very easy. I also normally take openings on a day-by-day basis, so if I don't need the money, I don't need to go to work on most days on which I really don't want to.
Cons
The salary is low, and in the state and school district I work in, it is only about minimum wage. Substitute teachers might make more, though, as might people working in other districts. As openings can be put into the system at any time before the school day, including the previous afternoon or earlier, I usually wake up early so I can check the openings that day. There aren't always openings, so work is essentially not available on some days. It is available on most days on which I check diligently in the morning, but having to get up early and check that is annoying. ESS also didn't really give us enough training for our jobs in my opinion. This made the hiring process easy, but I felt very unprepared at first. On many days, the teacher or other paraprofessionals did most of the work and did not delegate much to me, which I guess some people might consider a good thing. But I think that it might be because they thought that I was unprepared.