Ventajas
Good compensation was my primary reason. I received a healthy weekly paycheck. However, my job was salaried and I did a lot of overtime, but there wasn't any compensation for that. One of the other reasons I chose to work at Merck was that their current research areas are in fields that I eventually want to get into, so its helpful to start out in a similar field. Also, because its a huge company, you can move around a lot within the different deparments: antiviral, RNA, drug metabolism, clincial trials etc. This also makes it a good place to work.
Desventajas
There are very few opportunities to advance within the different departments. You can be a master's degree employee and you might just stay at the same title for 7 to 15 years. They prefer to advance the Ph.D over the employees who've been working there for a long time. Also, people aren't that good at Merck. Some supervisors too ambitious to give credit to those under them. There's hardly and unity or sense of community within Merck. Also, you need to be careful of your money. Merck will really encourage their employees to invest in their stock. Stay away from that, unless you really like Merck.