Pros
Good solid company overall with plenty of work on books. Have long term frameworks with lots of different clients. This stood them well through the recession and this limited redundancies. Good people to work with in every office and most offices have several disciplines. There are lots of examples of well established teams with long serving staff. Some good processes which work very well when applied sensibly.
Cons
Not always the most dynamic organisation because it can get slowed down by its own processes (like any large company). This will stifle growth in the longer term unless they keep buying smaller companies up. Beyond zero (BZ) 'culture of caring' sounds good in theory but needs some re-invigorating to get staff enthused by it. The things (some) management teams come out with are so trivial they border on being comical and staff under them see it this way. If you go for interview make sure you ask about their Travel Planning policy because some teams use it as an excuse to control staff to the extent that it adversely affects their ability to spend time with their families. Try and avoid these teams. Management (in the small number of) poorly managed teams find ways to supress the free thinking innovators who know their technical area well. These teams then go downhill and staff leave.