Pros
PHS is an enormous company with many different landscapes. I can't comment on departments outside of IS, but in 3.5 years in the IS department, a healthy balance between work and play was encouraged. Partners Corporate offers a wonderful vacation package (3 weeks + 9 floating holidays + 2 weeks that you may "buy" at your "hourly rate"). The benefit package is also generous, partially reimbursing employees for computer purchases, adoptions, etc. and subsidizes commuting charges. I only paid 135$ a month for my 380$ monthly parking pass at the Prudential Center. It was a safe, low-stress work environment where more junior employees were mentored and given opportunities to grow. Additionally, there was much job security, as people tended to stick around for years, and the company seemed to keep them on regardless of their abilities and performance.
Cons
The downside of working at a very large company (40,000+ employees) is the red tape one has to wade through to get things done. Additionally, there are rigid rules in place that can't be bent to suit a situation. Employees with minimal years of experience are compensated based on that, NOT their contributions and value to the company, and employees often feel their welfare has nothing to do with their job performance.