Just being a student who has rotated significantly through the UMass program - here is some info, take it for what it's worth...
Good program, strength is the teaching of the attendings and staff. The program is split between three sites. UMass (University campus), UMass Memorial and Milford. Memorial is a big hospital in Worcester, but formerly a private practice type facility. Used to have its own residency program but now the two are fully merged. Many attendings there are not quite as good teachers. Many residents think they don't learn as much from attendings over there because the focus is on the office practice and getting things done, not on teaching. Teaching more comes from some subspecialty attendings (again, many of these are also former private practice folks who weren't in an academic environment previously, so it's somewhat different) and senior residents. Memorial is also a bit less stressful to work at. Nurses and ancillary staff are a bit more independent and the hospital is not quite so busy. Milford is more like a smaller, community type place where I haven't rotated but have heard about. Resident schedules at UMass and Memorial are fairly similar, there is a night float, admissions are basically every other day (either short or long call) and no overnights. Milford has overnight call. The residents work pretty hard but not as bad as other places I have seen. The teaching staff at the University campus is a lot more dedicated to teaching.
UMass medical residents - there are a high % of FMGs. There are a number of US grads, mostly these are UMass med grads who chose to stay in Worcester, and many are very high caliber residents who could have gone anywhere else. Most of the FMGs are also good residents, and a credit to the institution. There are also a few DO grads every year. Fellowship placement I believe is pretty good, some choose to stay here, some move on. The residents work well together.
And, of course, you get to work with UMass medical students who are far and away the best clinical students in the state. Patient population is very varied, you will see most everything here and in sufficient volume to keep you busy. There are always changes going on in the schedules so what I said above will probably change slightly, but changes are mostly done due to resident feedback.
As with most places, a motivated resident is going to do very well.
I can't really speak to the research aspect.