People drive so fast in the rotaries that their cars list visibly. Normal levels of Boston Driving are amplified considerably, because we all know that those who enter must yield, but nobody really obeys that. The truly unnerving part is that as you line up to enter, each driver coming by may still be in the rotary or may be planning to exit in the street right after yours. He won't signal. Turn signals are for the weak.
Nothing beats the thrill of riding a bicycle through a rotary.
You're riding at a third of the prevailing speed. You're barely a blink in anyone's eye. You see nothing behind you. And then you exit. That which does not kill the bicyclist only makes him stronger.