Pros
Amazing place to work. The people are experienced, collaborative, empathetic, and fun to be around. C5 has a great process nailed down, so this is a really good place to come to learn how to do agile software development in a thoughtful way. Projects are usually 3 to 6 months (and you're only on one at a time), so you get a chance to see a lot of different companies, code bases, and work styles. There's also a super strong learning culture, with lunch talks and classes, chances to work on tech you'd like to learn, and opportunities to lead as a mid-level developer.
Cons
A pretty strict "work-from-work" (no work-from-home) policy. It's good for keeping a cohesive culture, but can be frustrating for people who live far away, particularly when they're on a project with a remote team. More just something to be aware of than necessarily a con for everyone.