Pros
I loved working at Booz & Company and continue to enjoy working for Strategy&. In terms of the client work I think we have the best of both worlds: we continue to focus on key strategic, C-suite level issues for our clients and have the added access to a whole range of new clients and specialized expertise from our legacy pwc colleagues. Culturally we continue to resemble Booz most closely though that may only be my experience. Being a part of a larger parent organization has distinct advantages (bigger employee and alumni network, in-house subject matter expertise) but overall it still retains the smaller feel of Booz in terms of affinity between staff and closeness of teams.
Cons
I definitely lost a number of colleagues around the time of the merger announcement. People had a range of reasons for leaving - many of which were very personal. In total the attrition seems to be only slightly higher than the natural attrition in consulting in a given year (though notably these departures happened over much shorter time frame). It is very telling that attrition since the merger has been completed is much much lower (i.e., people were more disturbed by the idea of the merger than the reality). Other cons include more burdensome admin processes associated with being a part of a larger organization.