Pros
Challenge, variety, experience. The big professional services firms provide a great environment for anyone to get a wide variety of challenging client-facing experiences. It looks great on the resume because alumni are widely recognized as experts in the job marketplact. Also, the mandatory sales work gives a good base for selling yourself to other employers. No one works at a large professional services firm for the money, it's all about the experience and challenge and long-term opportunities.
Cons
Long hours, grueling pace. Everyone is paid on salary, except for the partners, who get paid for the money made in a given year. There is no reason for them to control anyone's hours of work. The official party line is work-life balance, but it will never get you anywhere. Senior managers and partners easily work 60-70 hours a week, all year round.