I don't think there's any question that the MGH/McLean residency is more demanding than most (by an order of magnitude, in some cases). However, among top tier programs, I don't know that they work that many more hours. For example, Longwood residents seem to work just as hard, as do the residents at Hopkins, and maybe a couple of other places. Additionally, one of the 4th years at MGH said that she was surprised by how many MD/PhD's match there, because there is so little time during the first 3 years to engage in research. In her opinion, it seems like more of a clinical program. The second year and part of the third year are pretty brutal.
That said, I found most of the residents to be very happy there. However, I always take "happy" residents with a grain of salt. Different things make different people happy. So a number of them said they were looking for a "rigorous, well-structured" training. Plus, I don't know how many are wooed by the prestige factor.
I think the traning at MGH is outstanding, and depending on who you talk to, the "education to service" ratio is high. But yeah, if you go there, you should expect to work your butt off.