Pros
You'll be exposed to some of the best design minds in the industry, working on incredible projects all around the world. They pride themselves on being relevant and put huge emphasis on remaining current with design trends as well as technological advancements. You'll be pushed hard and you'll be involved in many projects in rapid succession. You'll be trained on multiple software packages and be supported by outstanding BIM, IT and HR departments.
Cons
High project turnover leads to a lot of work, but if you can set clear boudnaries and expectations this can be minimised. There isn't huge opportunity to grow and develop into more senior positions, these are generally reserved for a select few who really shine, and a more 'granular' approach to team structure could be useful.