The BMC is a fairly young residency, and it's in the Berkshires where the location is not ideal, so it's not surprising that not all the spots filled. However, if you can get over the location, the residency is fantastic. I rotated there as a medical student and found all the faculty to be amazing, very knowledgeable, wonderful teachers, and very supportive. The didactics were also very enjoyable. All the residents were happy and friendly and the environment as a whole was awesome. I have heard, though, that since they scrambled in 3 out of 4 interns, that the intern class isn't so great.
I didn't apply because I have very specific geographic requirements due to family, but it is a wonderful program that where I had my epiphany of wanting to be a psychiatrist. As a clerkship spot at UMass, the BMC is a well-known in our school as a spot for one of the best Psych rotations.