Avantages
Talented and creative coworkers. World class education focusing on problem solving skills and technical understanding. Great student culture (undergraduate). Tremendous opportunities that are easy to take advantage of, due to the huge and easily reachable pool of brilliant thinkers and world-class researchers (simple enough to schedule an appointment with a professor and discuss your idea). Great opportunity to network for the same reason.
Inconvénients
Recent events resulting from the recession have shown the disconnect between the administration and the students/research community. Your experience depends a lot on the characteristics of the group you work with. I've worked with professors who are very quick in giving very strong feedback on how you are doing or can improve, and also with professors who don't communicate at all and get upset afterwards when I didn't do what they wanted. Compared to the cost of living in Boston, graduate RA stipends are a joke.