New York City has been closing Park Avenue to cars from 72st to the Brooklyn Bridge on Saturday mornings this month. My wife & I rode our Bob Jackson tandem on it yesterday. Got on Park Ave. at 71st St. around 9:30 & rode south & over the Brooklyn Bridge, then north to the Williamsburg Bridge, over that, then wound our way through the LES to the Village & back up Lafayette to Park & 71st. Mostly fun, lots of bikes, though less crowded on Park than I'd feared. Some of the construction zones were a bit hairy due to narrowing the street and some considerable variation in riders' speed and skill.
It was certainly worth doing: any city-sponsored bicycle-centric event should be supported, and it is always nice not to have to deal with cars for long stretches of time. The police and volunteers were well-trained, polite and effective, and the vibe was calm and upbeat. Definitely not an event for paceline types or scenery junkies, or folks who hate crowds, but for building up urban riding chops on a tandem with minimal dues for screwing up, it suited us just fine.