As a substitute teacher, it's hard to rate the agency since your work is at schools with different class levels and topics. There's no much I can say about the agency management since I barely have a conversation with them after the training.
The role as a sub is mostly easy when permanent teachers leave you with all the materials. In a normal day basis under this circumstances, your biggest concern would be to keep the kids in a safely environment managing the classroom behavior. But, there can be occasions that (for emergency or other reasons) you arrive into a classroom without lesson plans or materials for the day. This are the hardest ones. Because of this, I started to carry on my own bag with materials, including sharpener, pencils, reading books, etc. This is a must if you plan to stay as a Sub for more than a couple of days.
Flexible schedule, you choose the school and assignments that you like.
Hard to guess where you are going when you don't know the school or admins, Sometimes you don't have break because in some schools the teachers have to lunch in students table.