Pros: great resources, lots of really bright people
Cons: little support for junior, non-tenure track faculty
For scientists, UMass is a great place to work if you become tenure track faculty with a start-up package. For those who work their way from post-doc to faculty the road is much more difficult, the job security far less stable, and promotion quite difficult. In large part this is because without a start-up package you do not immediately generate the papers required to get the first grant.
My co-workers have always been great, but my supervisor has shown incredible lack of professionalism, clarity of expectations, and absolutely no mentorship. I know this is an aberration, as I have many friends and colleagues who really appreciate their mentors.