Pros
I worked in one of the smaller teams and made great friends there, as well as having approachable partners who constantly sought honest feedback for themselves. The work was interesting and challenging, the deals were often headline (business) news, and if you prove yourself you can take on a lot of responsibility quite early on. The pay is good - although you could earn more for similar hours at a US law firm, and the culture was becoming more "US" when I left (target hours, bonus percentages etc). Good support services. despite quite a lot being moved up to Manchester. The experience you gain and the reputation of the firm means you are well placed to seek an in-house job later in your career if that is the route you wish to travel.
Cons
The hours can be long but that is to be expected, so ought not to be a surprise, and you are paid accordingly. There is a hierarchical feel throughout the firm, and promotional decisions were as much political as they were meritocratic. The firm has initiatives aimed at "diversity", including womens' groups and leadership courses. However, behaviour I witnessed in practice and information I gleaned from colleagues with children suggested that the goals were not being achieved.