The office at Booz Allen where I interviewed was in the process of hiring a significant number of people, so the interview was a bit of a cattle call. About a dozen of us were herded into a big room, and every half hour for an hour and a half the hiring managers would rotate around the room and talk to a different candidate, speed dating style. Every one of the Booz Allen side started their interview by apologizing for the process, as well they should. Maybe this is an efficient way to hire, say, fast food workers, but not six figure professionals -- it was a bit off-putting.
That said, I was very impressed with the caliber of the Booz Allen technical staff. They are all friendly, smart, and passionate about their work.
The HR staff was not quite so impressive -- they all seemed very inexperienced, and did not have a good system in place to manage such a large volume of interviews. For example, I got three different, pretty much identically worded emails from three different people on the HR staff, notifying me of my interview date/time/location and asking me to confirm that I'd be there (I dutifully replied to each one). Before I got an offer, one of the HR people called to feel me out on how much I was looking for, but couldn't even describe which job they would be hiring me for. In fact, he seemed not to have even considered the possibility that I would ask.