Pros
Unbelievable opportunity to learn, develop skills, collaborate with teammates, partner with clients, learn about different industries, etc. PMSI is the best example of a truly "learning organization" I've ever seen. Senior management is available to employees, genuinely cares about employees and clients and consistently does the right thing. Jack (the founder and CEO) is an excellent role model for anyone looking to start their own business. The tools and processes in place are well thought out and scale-able. The best part is that management is always open to suggestions so even if something isn't working for you, they will take the time to listen to feedback. For the most part, everyone at the company honestly likes each other and gets along well on a professional level. Truly, this is a great organization.
Cons
The other commentator mentioned that it can be stressful at times and that's true - but no more so than any professional services organization where you're serving clients. You will definitely have clients that you like more than others but the great part about how PMSI structures their account team is that you're never alone in dealing with a difficult client or a difficult client situation.