I don't mean what we yanks call "randonneuring" and what other anglophone countries call "audax," I mean the original origin of the sport, audax riding such as that described here: http://www.rusa.org/newsletter/04-01-07.html. Common pace (22.5kph), long-distance group riding. I've done as much as 1000K with common riding companions, but it wasn't truly an audax ride. And 32 of us tried to ride audax-style on a 200K this past weekend, but in the end only 8 stuck together for the whole ride. So has anyone ever done this? What's it like?

A Fleche is similar to Audax riding. That's the closest I've come.

i'd like to, but keeping pace sounds like the hardest part. i like the idea of 22.5 km/h (or whatever the chosen pace is), but it seems like you have to train with the same people in order to be able to finish together.

all of the rides i've done have been in the "allure libre" style, and it sounds like that's what most of the US randonees are like.

some of our more seasoned riders usually end up in a big group, and finish in the same group (10+ riders), but i'm not sure if that's considered "audax" or just riding together..

Audax Australia will be running their first UAF-homologated Audax rides (also known as Euraudax) next year. Ask again after I've ridden a couple of them.

The Union des Audax Français organize and homologate (since 1904) brevet from 100 km to PBP or more (Paris-Athens (2000 km) in 2004).
Our philosophy is "start together, finish together" and to be a school for the long-distance rider.
If the "Allure Libre" are call "Audax" in some countries, it's in reference of the "Audax Club Parisien" (but ACP dont organize Audax Brevet since the clash between Audax and Randonneur, in 1921 !).

If someone want to organize Audax Brevet in USA or create a "Union des Audax Américain", you may contact me (I'm the vice-president for cycling (there is also swimming, walking, canoe and nordic skiing with the same philosophy))

I've only done a couple of 100 km Audaxes and a couple of 25 km walk Audaxes. I liked it. It's very friendly, but since you're always riding in a bunch you have to stay focused much more than when you ride your own pace in a regular Randonnee. I'm going for a 200 Audax next saturday. I'll tell you more then...
By the way don't hesitate to ask Bruno (Dabzik) for questions, he's got a huge experience in Audaxing...

[...]So has anyone ever done this? What's it like?
The description in the quoted portion of the RUSA website remains quite accurate today. If you can bear with some French, you may want to read my report of the recent Paris-Bruxelles Audax (http://abeille-cyclotourisme.chez-alice.fr/souvenirs/2008_paris_bru_paris.html) organized early July by the Union des Audax Français as a 600 km-Audax (35 hours sharp), together with the return trip organised as a trait d'union Européen. That was fun. Three motor bikes opened the way and kept the road always open in front of us: never stopping even at red signals is a welcome comfort. Sleeping midway (approximately after 311 km, in Verdun) is magic. Even if you don't like audax riding all the time because it effectively restricts your freedom, audax riding is really an outstanding experience for learning how to ride long distance and sharing the burden of the ride with others. These guys are great randonneurs and know what they do. That may be because they are over 120 years of age ... :)

Now, on the matter of "the fastest can't support the slow pace", this does not (not) happen. The strongest and most experienced are inevitably picked up by either N°1 or N°2 to do their share of riding in front, and this is no cup of tea, because you must sustain the pace, event against a strong wind, for over 25 km non-stop. Also, the strongest tended to concentrate at the end of the group, not really in the group, and chatting together. Audax style is quite chatty indeed: not for die-hard solo riders

... And don't miss the pics of dabzic (here, N°2)

Pics of the Audax of the century of the first 400 km (http://picasaweb.google.com/bdanielzik/400AudaxDuCentenaire)