Pros
Competitive-to-great pay; excellent benefits; on-site gyms, coffee shops, restaurants; beautiful facilities. I have the best coworkers I've ever had. Smart, driven, funny, and hard working. Interesting problems to solve and cool people to solve them with. The feeling of mission -destroying cancer- is very real and something that brings a lot of highly motivated people to work here.
Cons
Your experience will vary a lot depending on which group you land in. Some are tremendously "vertical" and dictatorial, some are much more relaxed. There is also a lot of bureaucracy, as in many 20,000+ person organizations. Work/life balance will also depend heavily on your department. I've had a great career so far, but advancement/promotion opportunities are scarce.