Pros
Good benefits, facilities, office spaces and competitive pay. Casual work environment with no dress code aside from safety-related equipment when working in the labs. In addition the company is involved in every stage of the drug development process so there are a lot of opportunities available for people with a wide variety of skill sets and expertise, including STEM but not exclusively in the sciences. The all-in-house approach also means that Regeneron stands at the cutting edge of the industry and often involved in fighting diseases that are in the news, from Ebola to COVID-19. I have said before and do believe that working here I'm doing something that I can be proud of.
Cons
The company culture can vary wildly between departments, for better or worse. I've worked in departments that were wonderful and supportive and fostered a great sense of comradery, and other departments where office politics are inescapable and advancement is determined more by popularity than merit. The other big con, maybe bigger depending on the department, is the lack of flexibility with working from home. Since 2021 the company has been trying hard to roll back the flexible arrangement they created to combat the pandemic; bringing everyone back to site more or less full time regardless of need and despite the fact that the pandemic proved that essentially all roles in the company could be done completely remotely. Complaints about this senseless, pointless and unpopular "return to normal" are dismissed by management and the senior leadership team. To argue their case they repeat endlessly the same platitudes about "fostering an office culture" and "spontaneous hallway conversations", or whatever else they can think of that is entirely qualitative and therefore cannot be refuted with any hard evidence. This insistence on doing away with the only positive things that came out of the pandemic for the sake of pretending it never happened has already lost the company some talented people, and unfortunately that might need to happen to convince management to get with the times.