Pros
Great health benefits. There are lots of opportunities to learn in the form of seminars, grand rounds, and corporate programs. The employee resources department seems to be supportive of employee concerns.
Cons
This is highly dependent on your department and group, so take my comments with a grain of salt. My experience was not positive. The director of our research group, some of the senior members, and even junior faculty members cultured an environment of dishonesty when it came to publications and idea ownership. Active subjugation of group members was common. There were also instances of junior faculty talking out of both sides of their mouth, undermining credibility of junior group members. All said, if you are considering a position at MGH/Harvard just listen to your intuition after visiting and interviewing. The MGH/Harvard name is big in the field, and I ended up basing my decision on that. I wish I hadn't.