I've been with the company for a little over 5 years. Generally I enjoy working here, the salary is very good for the area I live in. However I recently found out that the department I work in has hired males and females at different starting salaries a few years ago (men have higher salaries than their counterpart women with same education, background, age and experience). For the most part (salary excluded) I believe women are treated fairly but management is male-dominated and not diverse, which makes it tougher for women to "network" with them. Most of the few females that are in leadership roles are there based on connections they have (family members in higher positions) and not based on merit or leadership skills. Others are clearly put in leadership roles because they are easily manipulated by the higher management and fail to have a strong personality that protects the interests of their own team.
Gender politics coming into play varies by department and management, and is difficult to prove. I'd be more worried about overall office politics than any kind of discrimination based on gender. From what I understand, maternity leave is a joke. You can get paid for some of it if you have Aflac and don't sign up for it when you're already pregnant (it cannot be a 'pre-existing condition').
Job Satisfaction Level
As long as you don't expect special treatment for being a woman.
Get involved in the Merck Women's Network, network up, find a sponsor, connect with a mentor, do things that might scare you, be on the offensive when it comes to career development (i.e., don't wait for someone to come to you for opportunities); ask for what you want.