Pros
Working at Test Double has been overall really amazing. People within Test Double are universally great to work with, and there's so much to learn from the people you work alongside. Everyone is a high level expert on something software development related, and the wealth of experience to draw on and learn from is immense.
Cons
The biggest upside is also the main downside: moving from client to client can be hard. You will be exposed to lots of new companies, codebases and processes, but you will also develop personal relationships with the people you're working with and then likely have to move on to new projects and clients... If constant change isn't something you enjoy then it can be a little tiring to always be stepping up to new clients and challenges, however that's really the case with consulting in general and not something that is specific to Test Double.