Pros
Weekly pay and there are benefits that can be purchased. You choose how many days per month you wish to work, but that's not a function of ESS. All school districts where you might substitute teach allow that, irrespective of whether they have contracted with ESS.
Cons
Benefits are not strong; ESS charges a $1 garnishment fee in order to pay for child support. I've never seen that before in any company, ever. Lame. Did not get paid for attending a mandatory meeting, which is against the law in Texas. When informed, the ESS HQ employee didn't care, and did not address the question. There is a 401K, but it's lame too. It does not allow for participation until after a full year of employment. Even then, a contribution by ESS is arbitrary.