I did an away last year at MGH. Basically, the comments made about this program in other threads are accurate. Incredible hospital with an insane labyrinth of OR's performing very complicated cases on a daily basis. The staff are top-notch and highly motivated. There are a myriad number of reasons to be at MGH, and it mostly comes down to whether or not you feel like you would fit well into such an environment.
Regarding the away rotation specifically, I worked 5 days a week typically from 6:30am to whenever, taking optional Q-I-forget overnight call. I personally spent a couple nights on the floor of their library because all of the other on-call residents were female with no additional call rooms. The month is tailored to your interests- I spent time in Ortho, Vascular, Neurosurg, OB/GYN, Gensurg, and Same-day surgery. The residents and attendings are great teachers and involve you as much as possible.