Pros
Great opportunity to get exposure on high profile cases and can be quite exciting to watch those cranky partners in action. One thing tho. Please throw your 'work-life balance' dream out of the window - Not going to put this as cons because this is probably the same for all law firms. But what do you expect, you are paid to learn!
Cons
There is NO STRUCTURED pupillage (even if you are told so in the interview) and you will need to be proactive to ask for work from LAs or partners to try out more area of laws within the firm. But, most probably you wouldn't have the capacity to take on other work from other departments once you are primarily assigned to one. So when you were asked to choose your preferred department before you start your pupillage - choose wisely. "Pupils of Shearn make wonders" - you will be expected to produce work by hook or by crook. No one care if you die half way. Hierarchical as usual big firms. Be very careful with your words and actions. Once you plant a bad seed with any LA or partner, you'll have to endure the bad fruit for your remaining time there.