Pros
Great work culture, give you the opportunity to work on potentially life changing project for patients health and doing it with brilliant colleagues. The company respect the work/life balance and you feel valued, even at the lowest rank. They give you also the opportunity for growth and are not afraid to give you the lead on projects, even if you have minimum experience. It gives you the opportunity to make mistakes and learn to be a better scientific. As a big company, the whole structure feels more solid and things move more rationally and in a smart way. Merck also try meaningfully to diversify its workforce and I believe it's a plus for every company. Benefits are great and I can see myself working here for many years.
Cons
As every big companies, the promotion system is slower and it will take more time to get a better position. The daily life benefits (like food, drinks and other) are not really there.