Pros
If you're just starting your career, it's a good place to learn the basics and start building relationships. Some of the people here are smart and nice if you get into the right group. However, to keep your career healthy, don't stay for more than 5 years regardless of what they promise. There are better companies to continue to grow and develop.
Cons
After about 5 years you'll hit the glass ceiling. Even if they continue to promote you, learning will slowdown. Management at Merck are ignorant. For some reason after 5 years people become hooked or afraid to leave based on some misguided concept that they will loose great benefits. All I have to say is there are a lot of better companies that will teach you more making you more marketable.