I’m writing this from my (stable) Nokia N900 running at 800Mhz (stock speed is 600Mhz. See talk.maemo.org for overclock details). The question on every-body’s mind is: is it faster? In a word, yes. Very much so. Things like the camera application which used to have notable performance issues (taking photos, switching to video mode, frame drop in video recording) are now butter smooth. Playing music via the native player, mp3s or oggs, is also now noticeably faster. Whereas before the device as a whole would chug a tiny but during audio playback its now not noticeable.
Finally the final major improvement ive noticed is with youtube videos in the native microB browser. YouTube pages used to load a bit slow and have some issues redrawing the page with a video playing. Playback is now flawless and moving the page with a running video is definitely better (though still not 100% smooth).
All in all: I’m never going back!
Update: I’m writing this after 1 full day of usage and, at least thus far, have noticed no change in overall battery consumption. My batterygraph unsurprisingly doesn’t look, on the whole, steeper than usual. The common theory is that there would be no change thanks to the cpu governor and aggressive down-clocking.