Pros
Relatively relaxed environment, nice offices, friendly/noncompetitive coworkers, principal investigators are major players in their fields. It's a fairly small place so there isn't as much bureaucracy. If you're a student you have much more independence than you would at other places, and will generally be treated as someone who can contribute. Albuquerque is affordable and has great weather, with a nice arts culture.
Cons
Working at MRN ends up being a dead end job for a lot of people. There are non-tenured research scientists (with a PhD or masters) who have worked there in a non-leadership position for over 10 years. There are some people who later get good jobs in industry too though. If you are ambitious and looking to lead, there may be more fast-paced places for you. If you want to focus on a family or don't have major plans for promotion, MRN is perfect. There isn't a whole lot of coworker interaction or a company culture, although there are some holiday events and etc.