Did this yesterday with my wife on our red tandem. 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. Bikers were very polite for the most part; pedestrians less so. Lots of understandably impatient walkers trying to cross Park Ave. There were no traffic controls on the streets that were blocked off, so cross-town walkers were stymied by the masses of cyclists. Saw a number of near misses. Almost hit a young beauty as she crossed, totally oblivious to all but her cell phone.
The Brooklyn Bridge was a drag, as the walkway is really narrow right now, and every ambulatory human and dog in NYC was on it. Brooklyn was empty, as all the residents were on the bridge, so we had a fine ride through industrial/residential melange that is the Brooklyn waterfront. The Williamsburg Bridge was lightly used, so the ride over it was a pleasure. The return trip was smooth. I started moving quicker despite whimpers of terror from my wife, who told me later she closed her eyes a lot. The hipsters had rolled out of their squalid garrets by then, so we were accompanied by a number of fixie-ridin', messenger-bag-totin' young folk. One did a couple of respectable trackstands on his new Bianchi, and another sported a tee shirt that said, "DROP BARS, NOT BOMBS". His own bars were short and horizontal, so he wins the Irony Prize.

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.