Pros
Mercer varies greatly depending on which area you work in, and where you work. The UK has a good work/life balance ethos. Expectations are not too high, so it is possible to succeed without killing yourself in the process. Full of smart people. Some teams can be very social and supportive. Many career streams offer opportunities for international travel and work. There is a entrepreneurial sub-culture and it is possible to 'start-up' new areas within the business. Common to be given a chance to develop new skills, or take on different projects - and learn as you go....
Cons
... however, Mercer is also full of people who have worked there forever. Early in your Mercer career you are given lots of opportunity - but that tends to peter out the longer you stay no matter how successful you have been. Once you are pigeon-holed, it is near impossible to escape. There is not a huge amount of challenge, and so it can be very boring if you are looking for a super-career. Some areas are frustratingly conservative. The organisation on a whole is very slow to change and adapt, not very dynamic. Senior leadership lack vision and inspiration. There is not enough "good" churn in its people - it tends to lose the talented ones and keep the the ones who become institutionalised.