Not my sentiment but that of Johnathan Nightingale, the Firefox security interface designer no less who has gone on record as saying that "we didn't want to put in something that was half baked" when talking about how the privacy button might interact with websites and mashups alike. Which might explain why the feature has been removed from the final version of Firefox 3 which is due for release real soon now.
It's a real shame in my opinion, because the privacy button provided that Holy Grail of web browsing: a non-sticky surfing experience that is integral to the browser client rather than coming by way of a third party add-on.
It would have worked by disabling caching and cookie downloads alike, as well as history logging and form data saving during any given session. It would say directly to the browser, as Nightingale himself admits "I would like what I'm about to do to not be logged anywhere."
Or at least it would have done, had it not been removed from the release version.
Apparently, Nightingale says it "touches a lot of code" which could be problematical. So, on balance, perhaps a stable browsing experience is preferable to a private one. Well maybe, but it's a damn close call for a lot of folk.