Pros
Reasonably good reputation that ought not to do the CV much harm. Almost automatic promotion / career progression in early years when joining as a graduate. Huge graduate intake means it is relatively easy / uncompetitive to get in. Good client base and some varied and interesting work further down the line.
Kontras
Conservative firm with the stereotypical accountant image and culture (boring, poor social skills) tending to pervade all service lines to some extent. It varies from place to place but most people who work here are conformist and not particulalry interesting, sociable or cultured. "Diversity" means a few candidates from BME backgrounds rather than any true variety of people. Many of the graduate intake are tiresome individuals who did not give sufficient thought to their career choices until their final year of university. Unqualified for proper careers and not quite good enough for banking they then select a career at a Big 4 firm as Mummy and Daddy have heard of them, it cuts enough mustard with their peers, the graduate marketing materials were reasonably slick and it just seems that bit sexier than that grad scheme at whatever-it-was blue chip. Having secured the job they really think they are doing something prestigious and worthwhile rather than entering on the least competitive graduate scheme going and destined for several years of tedium. Real disillusion inevitably sets in a bit later when they realise what audit actually involves and that being an accountant is profoundly uninteresting to any body they might meet at parties. At this point they tend to leave and find more interesting jobs and get some of their life back, hopefully before it is too late. This accounts for the huge turnover / attrition. There is little point in staying too long as the reward scheme is purely geared towards a "jam tomorrow" possibility of becoming a partner (a possibility that is fairly limited, for most). There is no such thing as any equitable share of the profits on the way and your charge-out rate will be entirely decoupled from the cost to the firm of employing you. Rather it is designed to gouge from clients as possible and maintain the bloated remuneration of the partners.