Vantaggi
Many of my coworkers became close, personal friends. Honestly, I probably stayed a couple of years too long just because of that. There are also lots of resources available to you that you don't get at smaller companies. For the most part, walking into a room and saying "I'm from Booz Allen" still gets you instant credibility in a whole lot of places.
Svantaggi
Despite being a seasoned professional in my field when hired, I quickly realized that I was brought on to be a very expensive intern for my vice president. My days consisted of making power point presentations, excel spreadsheets, scheduling meetings, and listening to other people brief my material. It took me a couple of years to be given even a modicum of the responsibility I had for years prior to joining the firm. Once I had it-- and excelled-- I was repeatedly promised promotions that never materialized.