Pros
ESS is a very flexible. They don't dock you for canceling although no doubt if you abuse that responsibility your standing in the company would change. You can choose an assignment and you will instantly have it. No waiting lists or wondering if some preferred candidate has first dibs. Their support is good although depending somewhat on which district you work in. It can be very rewarding as you have many opportunities to make a difference in student's education even if it is only one class or one day.
Cons
The pay is mediocre compared to what is expected, especially if you really put out an effort to be the best you can. Often you will sign up for one assignment and when you get there the school will place you in something very different. Usually though, it ends up working out in a positive way. They can cancel your assignment at any time with no reason. Usually this doesn't happen. At the same time you can do the same to them. So what's fair is fair.