Pros
Merck is a great place for benefits and recognition (such as lunch passes, movie tickets, award dinners, and just notes to say thank you), in my department at least. They recently increased the number of vacation days we get per year. They also just started a more flexible work week where you can work from home part time, have a 4 day work week, or other arrangements. Also, since the company is so large, there is a lot of opportunity to move around and try new positions and become more well rounded. The people I work with are the best! They keep me coming back every day with the family-type atmosphere.
Cons
Right now things are in a a transition state -- we are implementing SAP, which in the end should make day to day transactions easier, but in the mean time is creating quite a stir. Many trainings and "hyper-care" stages are on their way! Senior management is sometimes inaccessable, in a way they seem to be making decisions on their own and there is a very subdivided oganization, but I know there are strategies in place to make this better, so I am optimistic for now.