I'm curious about your opinion of most "Boston accents" in movies and television programs. Do you think they're typically pretty good? pretty bad?
I'm not a native like you, but I did live in the Boston area for a few years, so I have a general idea what the different local accents sound like. I thought John Ratzenberger did a perfectly plausible (albeit comedically broad) accent on Cheers. I don't watch a lot of movies or television, so the only other attempted Boston accent I can think of just offhand is Tom Hanks' in Catch Me if You Can. I thought it was perfectly credible for the most part, but slipped badly every now and then, which I found distracting. Since the character's background was not a plot point, Hanks would probably have been better off either skipping the accent or toning it down -- or re-recording some lines during post-production.
There's a wide range of opinions online regarding both of those performances (including some people who thought Hanks was attempting a Chicago accent!), but my favorite is this (from the comments section of this page):
That cracked me up! And I think she's correct, really.
Most sound too forced. I worked to lose my accent when I went to college in the south (University of Florida). It comes out to play only when I'm tired or I want it to. Still, it drives me crazy when poorly done. I agree with you on John Ratzenburger. But he is from Connecticut and probably heard many real Boston accents growing up.