Pros
Work from home. Not too taxing. Good if you like a fairly solitary work experience. Helps if you've got a good grasp on grammar and writing. You can choose when you are available to work, though you won't get everything you ask for. At roughly $13 per hour, the pay is not high, but I consider it barely acceptable for the benefit of not having to commute or buy work clothes or buy lunches, etc. A couple of dollars more per hour would seal the deal and keep me around for the long haul. As it is, I'm just doing it until something better comes along.
Cons
Scoring guidelines can be confusing, and your "mentors" are well-meaning but often give contradictory guidance in scoring student prompts. Your supervisors change each day of scoring, and you are never provided with any contact for anyone in management to air concerns or questions beyond your immediate Scoring Leader, who is almost as low on the totem pole as you. If you are ever trying to get specific answers from the company about hiring or maybe adding a new test to score, there are no clear ways to find anyone to contact to get questions answered or to get any guidance. There is little effort to get feedback from online scorers - a site like glassdoor is much more accessible to try to provide feedback for the company than any mechanism the company provides or invites. Work can be mind-numbing. By the end of an 8-hour shift it can be hard to focus your eyes on the responses.