Pros
Company is flexible with family / working from home. Pretty generous with vacation and flexible if you need a day off for having worked longer hours on a tough project. Lots of smart folks work there, but it's not too portentous a place. Good health benefits, and benefits in general.
Cons
No in place promotions? In the past few years, times have been tougher and they've been skimping on training...as an IT professional staying up to date on what's current is crucial. Getting management to send developers to training / conferences on new development technologies is like pulling teeth. Management doesn't want people developing applications for scientists, rather they want to buy global solutions for nearly every possible situation, which is a recipe for disaster. For scientific research software development that is done in house, there's no emphasis on code reviews / writing good code / applying any development methodologies as a team; a lot of code for lab and science-specific apps gets done by single individuals (this might not be the case for teams writing clinical applications or company infrastructure applications, like internal websites). Be careful about what you're getting yourself into if you're into writing software. I'm not saying that all positions at Merck are bad for IT, just be careful to ask questions about the position before hand about working with a group or possibilities for development-specific training if you're into software development. More politics and in-fighting than you could ever believe. Place is rife with bureaucrats at all levels; some folks seem more interested in playing games than getting their job done, although this is probably typical of many large companies.