It has noble intentions I don't doubt, but there are a hardcore group of cyclists and longboarders who seem to use it to antagonise other road users. Definitely avoid if you hate hipster fixie/fakenger types.
Can be vaguely fun if you are with other people or you are around one of the sound systems playing decent music. Don't expect to cycle particularly fast or anywhere logical. Invariably ends up heading north of the river so a bit samey if you do it more than once. Probably best enjoyed somewhere in the middle of the pack so you don't have to be around the people deciding which way to go and you don't get cut off if the mass splits.
The police take a fairly strong dislike to it and I don't blame them to be honest (although they can't do much about it since they lost the high court appeal about whether it is a procession or not......except on special occasions like the olympics opening ceremony kettle fiasco).
Facebook is probably the best place to find information about it now because the website doesn't seem to update any more (probably because it isn't meant to be a planned event).