First of all, Merck is a fantastic place to launch you career. You get exposed to the good and bad at working at a Fortune 500 company.
Cons
As you move into the middle management layer, things get very political, there are a lot of alliances. To be successful you have to be wary of the connections and navigate the politics.
Merck hires the best. Very talented and dedicated workforce. Senior Mgr is trying to make things better for employees and recognizes the issues. Very flexible telecommuting culture.
Cons
IT people in my area work on avg 50hr/week. Late PM meetings. Highly charged budget environment. SAP and Schering PLough has impacted morale which is very low. People are just burnt out. Maybe the same everywhere. Wave after wave of reorganizations and layoffs and moves do not help. Conscious effort to move from inhouse to contract for many services (esp IT) leading to more upheaval.