Pros
3M is a great company with a long track record of developing new products and great employees. Everyone that works at 3M is generally happy, and have been with the company for a long time. 3M creates a nice work life balance, and generally promotes managers who have good people skills, and genuinely care about their employees. 3M has deep pockets, and can do somethings that other companies might not be able to do as far as discounting products, and providing technical and sample support which can really help a sales rep win in the field. Marketing also does a nice job of communicating with the sales reps. Overall, 3M really is a great place to work, and morale is generally very high.
Cons
When I worked for 3m they had just got done going through a lot of changes (guided by Jim McNerney and Six Sigma) that cut a lot of the innovative side of the business out of 3M. The effect that this had (in my opinion) is that a lot of the "value" was stripped out of 3M's product offerings. What customers have come to expect from 3M is that they offer new and innovative approaches to everyday problems that customers face when manufacturing products, or performing everyday tasks necessary to keep a business running. The new CEO, George Buckley, has been playing catch-up the past few years now in order to supplant the lack of innovation by acquiring other companies. Whether this approach has worked, and or will work, remains to be seen.