Pros
Merck is full of extremely talented people (which became even more evident to me after I left), and this will help to raise your own personal performance. In my department, promotions were handed out based on a combination of seniority and performance, though for higher level promotions, seniority rarely played a role. I felt that decisions on promotions were fair and highly transparent (got passed up once and got promoted another time); politics wasn't much of a factor. I believe that the pay and benefits at Merck were comparable to other large pharmaceutical companies and may have been slightly better than in most industries. Employees in my area were generally quite proud to work at Merck, though this had been declining in recent years. Perhaps the main reason that I stayed as long as I did was the fact that I was friends with so many of my coworkers. When I started out of college, there were 15-20 of my peers in my department close to my age. In my experience, it is very common for people at Merck to socialize outside of work. Not every company is like this.
Kontras
The fact that Merck attracted so many talented professionals was a double-edged sword - while it was great to work at a really high level and learn from the best, it also meant that you had to wait longer for that big promotion (if it ever came at all). Many in my department realized this and left for much more money/responsibility. It was common for an average employee (say ranked in the 40-60 %ile, my department used to be big on rankings) to leave with 1-2 direct reports and then be heading a department of 30+ within a few years of his/her departure. Another downside of life at Merck was that work/life balance was pretty poor. As more and more people got laptops in recent years, it got even worse, since you'd get ten emails on the weekend that needed a response before Monday. No, it's not i-banking, but I've worked for three Fortune 500 companies at this point, and I feel that the workload at Merck was considerably higher than at the other two.