Pros
Working with a wide variety of other companies means there's a lot of diverse things to learn- processes, tools, entire industries you may have never given much thought to before. There are career paths into training, accounting, project management, programming, IT, etc.
Cons
Middle management can have a "mercenary" mindset; because all work is a service to another company, there can be a lot of red tape and miscommunication. Corporate can have good intentions, but day to day yelling from clients can influence lower level leadership to make misguided short-term decisions. There's always lots of change, which can be good or bad. If you like being adaptable and solving problems, you'll do well, but it can be stressful when customer volume is high, or when something unfortunate happens out in the world that causes tons of customer complaints. Feels great when you can help someone though.