MGH CA-1 here. Interviewed at all three of the Boston programs, and absolutely fell in love with the MGH residency program. It's true that work hours are long at MGH, and that one learns to become autonomous very quickly (in the face of little tolerance for incompetence or intellectual laziness), but there is absolutely no other place I'd rather be.
Although MGH has strengths in clinical rigor (only Trauma I center among the Harvard programs) and academic research, the best thing I like about MGH is the people that you get to work with. Attendings are vocal and advocate for residents during operations. Senior residents take time out of their day to act as 'big brothers' or 'big sisters' to residents. Co-residents often switch call days so that peers can attend functions -- one day my chief resident actually covered my OR responsibilities for me to attend an important event.
I completely agree with macy's statement that 'The "best" program in Boston is the "best" program that fits your goals and personality.'. If you are hard-working, eager to learn, and want to be a rock star anesthesiologist, you should seriously check out MGH