The unfortunate part of this is exactly as you say--you first tried it in a bleeding-edge version of Safari, and it wasn't good until you updated to the latest nightly build.
That doesn't bode well for performance in other browsers like Firefox--let alone IE.
To clarify, it wasn't performance that left me wanting. It was still a usable Flickr browser. It just didn't live up to the headline until I tried the nightly. In other words: it degraded well, and this is actually a terrific validation of the extensibility built into the CSS spec. This isn't like the old days where you couldn't log into your online banking without IE. So I disagree: it bodes quite well, IMO.
That doesn't bode well for performance in other browsers like Firefox--let alone IE.