Pros
It all depends on what area of the firm you're in. In the Advisory business in Financial Services (FS) the calibre of people is very good. What counts the most is your experience, not so much your qualifications (e.g. uni degree/MBA). It is colleagiate, people are very approachable at all levels and willing to help. There's very good training opportunities, mostly in house. The client work can be fairly varied and you can get exposure to lots of different work in a short amount of time. You can work very flexibly so long as you deliver results (face time for its own sake is not required). I've worked in well resourced projects so far so the hours have not been crazy. In FS i have not travelled much - been mostly in London. I'm usually based on client site, not really the Canary Wharf office. It is small enough compared to the other big 4 that there are many opportunities to take on more senior roles and get promotions. The fact that processes or propositions aren't as well developed also means that you have opportunities to build a lot of your own work and ideas. It really all depends on who you know and who knows you and especially the reputation you make of yourself. If you love building relationships, get on well with all sorts of people, love solving problems and working with ambiguity, love variety and constant learning plus are conscientious and follow through on what you promise then this is a good place for you.
Cons
It can be pot luck on ending up on projects sometimes - a bad one can really make your life miserable (bad could be any combination of: difficult client or project lead, inconvenient location, uninteresting work, crazy hours due to tight deadlines). Promotion is a very obscure process and depending on the team can take place once a year or throughout the year, you can feel like it's a moving target as requirements change. Salaries are capped at seniority bands and therefore you need promotions if you want to earn more even if you don't want to necessarily be a "director" just want to be a specialist in your field without pressure of sales targets. the experience can be way different from team to team - you really need to choose the people over the work/sector sometimes. I have worked in more than one team and it has been like two different companies with their mini cultures! The partner in charge sets the tone. To advance you need to have a partner with clout and who's going places. It is a hierarchical place and sometimes the lack of the right job title can prevent you from gaining traction for a good idea. the partnership model means it is hard to move swiftly with an idea that requires resources across teams - it takes forever to get buy-in from more than 2-3 partners. Sometimes you need 20 partners involved and that just kills the idea before it is even born! maternity benefit not great - max is 14 weeks paid or so but they do try to help you get back to work which is a plus if you enjoy your job.