Pros
No supervisor (you are substituting for school employees).
How you go about doing your job is totally up to you.
You may choose to work anywhere from 0 to 5 days a week; your schedule is infinitely flexible.
The pay for teaching positions is great for the amount of work you are actually doing and the time you are there.
Cons
No benefits.
No training. Also, I was hired with no face-to-face contact with PCMI employees other than a fingerprinting session that lasted all of 5 minutes. I would want substitute employees to be subject to a little more scrutiny during the hiring process if I had children in the school system.
Certain job types come with low rates of pay (teacher's aides and janitorial positions are paid hourly and are significantly less per day than teacher positions, which are paid at a daily rate.
There is no guarantee that there will be work available on any given day (very few jobs are available during the summer, as school is not in session.
Depending on what school district you are working in, dealing with students as a substitute can be very difficult and stressful, even if you are good with children.