Great Brook Valley isn't nearly as bad as it once was (say, in the 80's-90s or even early 00s). Though I wouldn't expect a rental to be available there anyway, so it's kind of moot.

Main South has seen some revitalization recently too, though I'd avoid Main St itself (not just south, all parts) in terms of living on it. Being able to walk to it is fine. I do agree there are better parts of Worcester on average, but I wouldn't rule out a Main South rental at a good price without seeing the area first.

I'd avoid Main Middle more than Main South these days. Not because I felt in danger so much as it just having a sad atmosphere. Passed through the area on foot a couple times, and it was all older 3-deckers with everyone shut inside despite the warm weather, the front porches, streets, and sidewalks pretty deserted.