Pros
The sheer diversity of nationalities, ethnic backgrounds and languages spoken, the extraordinarily young average age of employees (including managers) and over half female employees means it is a million miles away from the old boys' club atmosphere prevalent in the City. People join the company straight from university and are trained up, where other companies would demand masses of experience instead. There is a culture of innovation and getting involved, wanting to help each other, being brave and trying things out (all part of the sometimes overwhelmingly American work ethic and corporate terminology). People who work hard, are talented and are resilient can go very far very quickly, no matter who they are. The quality, speed and professionalism of our work is taken very seriously, people pour the best of their intelligence and education into the work they do and, especially if making a major contribution for a major client, are often handsomely rewarded for their efforts, both materially and in terms of recognition. There is a very, very strong social aspect of working here, it is actively encouraged to make friends and do fun activities with colleagues, particularly those you share a team with.
Cons
You have to be willing to buy into the American approach to working life: work hard, be seen to work hard, be enthusiastic about opportunities to do more or different work, rather than being indifferent to them. Of course people in companies of all shapes, sizes and nationalities work hard, but in the American working culture, expressing your positivity and excitement is key - this is not a place to be grumpy, over-critical, lazy or obstructive. Salaries etc usually start quite low (but can grow fast).