Pros
Great people to work with, many smart scientists and engineers from all disciplines available for advice and collaboration. Many exciting technical areas of work. When the job reqs arent frozen, if unsatisfied at one place one would be able to switch jobs to a completly different area without having to leave the company (keeping all the benefits and vacations).
Cons
This might be true for any large company, but it is really hard to stand out for a raise. The promotions/raise system is set up in such a way to keep everyone below industry median pay at all times. Not very competative. Little reward for doing an awesome job or 'performing at a much higher level'. It's like communism, everyone is the same, which rewards lazy people and chases away great workers that are willing to fight and take risks (what 3M needs most now). It used to be that 3M paid their employees less but offered more intangible benefits like a secure job, flexible hours and a more freedom for research. These benefits are gone, and the pay stays low.