Pros
The work is generally challenging and most of the people are great to work with. It looks great on the CV and the pay is normally pretty good, although this can vary depending on whether you get a duff review.
Cons
No work - life balance. They expect you to earn your money, so you'll spend an awful lot of time at the office and probably a lot of time away from home. The laddering system used for performance ratings can be very hit and miss. You can find yourself getting a good ratings for not doing a great deal one minute and then very average ratings for working your arse off the next - depending on the project you're on. For promotions there is a real emphasis on time at level as opposed to whether people are actually any good or not. I've seen people who are extremely average get promoted ahead of others that are much better but haven't yet served their "time". At the more senior levels the Senior Execs can start treating you like a dormat.