I've been a long-time critic of what Critical Mass has become. I remember when I worked in San Francisco (1998-2000) and it was a small but active presence on the streets of the Financial District. I've seen it move from being a bunch of cyclists getting "in the way" to the violent "everyone sucks but us" mob-rule ride it often becomes these days. I agree with what it is supposed to be about (raising the visibility of cyclists, protesting the lack of accommodations) but not the way the message is delivered. On my BART ride into work this morning I was listening to a podcast and got a different take on it.
I was listening to the show Law in Action from BBC Radio 4. The first 10 minutes or so of the 10/14/08 show are on a case involving Critical Mass that has worked its way through the British legal system until now, when it goes before the Law Lords. The short version is: the 1986 Public Order Act organisers of mass-gatherings are supposed to inform the police about the event (like in the US) ; an individual who was part of a CM ride challenged the law and won under the "there is no organizers for CM" argument; the appealed and won on appeal. The general argument from the rider is that CM 1) is not organized and 2) there is an exception in the law for "customary"(regularly occuring) events which CM is. The general argument from the police is that knowing the route in advance would allow them to better serve the general population by rereouting traffic as with normal parades.
It is a very balanced presentation, with the presenter going so far as to take part in the ride and record at least a few moments at the begining, despite having not ridden a bicycle for years. This is one of the few instances of coverage of CM that I can think of in recent times that hasn't been trying to argue one side or the other. Additionally, the presenter is careful to mention possible negative ramifications of the case going for the MP, namely a potential chilling effect on other marches if the organizers are afraid the participants might stray from the approved route.
Whatever your take on CM I recommend checking out the podcast. The story is the first ~ 10 minutes, and includes interviews with riders, police, laywers, and law professors.