Pros
The people. My experiences at Booz Allen Hamilton proved to me that when you put a bunch of bright people together, anything can be done. My team provided a truly collaborative atmosphere, and encouraged everyone to take on team leadership roles, allowing them to learn not only from their client engagements, but form their peer engagement as well. The different forums and affinity groups introduces a good atmosphere for interacting with those with common interests. It provides a great arena for networking, and encourages diversity (both cultural and professional). Although I never had the opportunity to use it, Booz Allen has a Career Mobility initiative which helps employees explore different opportunities within the company if they wish.
Cons
It might be too big for some. There are so many projects going on at the same time, that it may get difficult navigating who is doing what.