Boston is having its first official summer of them, and this Sunday from 10-1 the Greenway will be open for non-motorized transportation and recreation with all kinds of events going on. Info here
In Cambridge, we tend to take our "Cyclovia" completely for granted- it's been going on for probably 20 years (or more?) and nobody really pays attention any more. Memorial Drive is just always closed on Sunday from sometime in April to sometime in November, from 11 to 6 or so. There's not really any publicity or fuss, it just happens every week in the summer. Cars deal with it, people take advantage of it, or don't, depending on the weather and their plans for the day. There aren't really any formal activities planned, although there always seem to be skaters with cones doing tricks, and lots of people have picnics, and recently there have been a lot more food trucks parked on the suddenly dead end side streets.
I think it's great that Boston, and other cities are getting in on the open streets bandwagon. I do hope that the idea that streets can be used for things other than cars is the beginning of a transition in some people's brains. I'm just not sure how influential this will be in creating safety for people using the streets when they're not closed off for some "special" "recreational" event.