Philly will be having its third one this year. The first one was a mess because they chose a street that was way to narrow (essentially three lanes because no parking was allowed) to support all the people who showed up. You could not safely ride more than 7 or 8 mph because of all the people with kids and dogs and strollers, etc. And there was no directional flow. People just rode and walked wherever. Quite tedious. I saw many near misses and even some cyclists fall. One woman appeared to be hurt somewhat seriously.
Last year was much better since it was held on six-lane street. There were also signs instructing people to stay right of the median in each direction. I think the same street will be used again.