Pros
I worked part-time for Cambridge Proofreading for ~3 years, and it was a great experience. They offer a very cool platform that lists available editing jobs with the details of each document, so that it was easy to discern whether a particular document was a good fit for my expertise and time availability. They would also assign specific jobs to me based on my subject-matter expertise, each of which I could accept or decline. The managers are very personable and supportive, and it was always a pleasure to interact with them. This company has integrity; for example, they paid me for the sample documents that I edited as part of my job interview. I'm aware that other editing companies cut corners with their workers, but in contrast, I always felt that the managers of Cambridge Proofreading were acting in my interest, and they always treated me fairly, respectfully and thoughtfully.
Cons
Not many cons; it seems that the pay could be higher, relative to other editing companies, but one of Cambridge Proofreading's selling points to customers is its relatively low rates. For the work that I was doing, I felt that I was being compensated fairly.