Pros
The biggest pro is definitely the people. The people that I worked with were all very hard working and incredibly smart. The biggest motivator for a lot of the staff were finding projects that could be challenging and present interesting puzzles to solve--I think that's one of the beset indicators of a great workplace. I also always felt that senior management did their best to staff projects in a way that suited both the junior staff's current abilities as well as their interests.
Cons
The work could get repetitive. It's also a consultancy so the amount of work could range from 60 hour weeks for months to weeks where there is nothing to do. It's a company based in London with satellite offices around the world so offices in the US can feel like they don't receive the same amount of corporate support or benefit that the UK offices receive. I also found that it isn't the type of company that pushes opportunities at its staff. The opportunities are there but you have to be proactive to get face to face time with clients, to manage projects, or to get on the more interesting projects.