Pros
Merck's benefits are excellent and cheap. Medical plans are heavily subsidized by the employer, with little out of pocket contribution from the employee (as compared to other Fortune 500 companies). Merck provides internal advancement opportunities. Work / Life balance can be good, but there are a lot of employees who routinely work 10+ hours per day at Whitehouse Station. As always, this varies by department / business. The Whitehouse Station HQ is a very nice facility with amenities (fitness center, health center, coffee shop, gift shop, etc). Even though it's no longer the go-go 80's, profit margins are still strong and cash flow is good. This enables Merck to invest in new technologies and make acquisitions.
Kontras
SAP implementation is a huge boondoggle. The company cannot seem to stay out of the headlines. Long-timers give off the sense that everyone should want to work at Merck, so they don't make an effort to attract new talent. Novel R&D doesn't seem to be a priority, new products will probably come through acquisition rather than internal research. There is a lot of dead wood at Merck, just waiting for their full pension to kick in. There's a lot of "Merck knows best" attitude.