Oh, here we go... I knew the Boston accent would come up sooner or later. Look, I live in Boston, was born & raised in Dorchester, and 90% of the 'Boston accents' you see on Youtube are of people faking a Boston accent.
A real Boston accent sounds like this:
http://www.youtube.com/watch?v=RbK4cL3QSc0
You see? Nothing stupid or over-the-top... just normal, sane English with a delightfully colorful dialect...