Vantaggi
One of the best things about working at KPMG's Boston office was the colleagues that I worked with - of whom I'd say 90% were smart, articulate and willing to learn and teach one another. Its definitely a place where teamwork is critical and you develop a lot of teamplaying skills, including negotiation skills, learning how to be diplomatic and working in close proximity with people who are leaders.
Svantaggi
Personally, the performance review and overall appraisal process always struck me as odd. No matter how highly reviewed I was by my colleagues and seniors (and I pretty much always made it to the very top tier), it always seemed to matter more how schmoozy you were with the performance review manager, since he or she was the person actually fighting for you at the time when it mattered (i.e. at the managers performance meetings). Also the traveling that they sometimes try to make you do is a little ridiculous. One year I traveled more than 85% of the time and not to glamorous places.