Pros
Good starting salaries, good training, good company name on your resumé. Merck is the best training ground for use as a stepping stone to other pharma/biotech companies who pay much better for seasoned workers. Make sure your boss likes you and will help you develop in your job. If your department head gets to know you, opportunities might arise, but don't step on your boss's head.
Kontras
Very political as you move up in the ranks. Very Type A anal retentive types. Sales and marketing are essentially cults. There were some male predators there in the past. Downsizing has made for a higher-stress environment. Overall, Merck hires the best people and does not develop them. Much favoritism, so if you are not a "golden child" you can jump to a better paying job at another company.