Smart people, terrific science, great atmosphere
Pros
Being a scientist at Regeneron is about as good as you can get. You have reasonable freedom to pursue questions of interest, terrific resources at your disposal and you're surrounded by very intelligent people all of whom are more than happy to collaborate and offer advice. It's a great team, the company is doing very well and, quite simply, it's one of the best places to work in science, period.
Cons
The only real con is the location, as the cost-of-living in Westchester is phenomenally high (especially if you wish to buy a house and settle) and this puts stress on lower-salaried employees. But the benefits you get are great and do make up for some of the cost-of-living concerns. Yes, there are periods of stress and pressure which come and go in cycles, but no more than you would get at any other job.