I tried it, and it was super fast, but if anything it achieved that speed by causing even worse CPU churn, so after a while oohing and aahing at its speed I toasted it, uninstalled and went back to Firefox 3.
Then I read this article, and the article it links to, and sure enough, in the short part of the evening I was using Safari 4 I had generated 170+ megs of data in a hidden, can’t-reach-without-command-line-fu location on my computer, not even in my personal user directory. (I don’t think it’s hidden for nefarious reasons; Apple doesn’t roll like that. I think it’s hidden because this cache is what the Apple engineers needed to achieve the effect they wanted, and they didn’t think you should have to worry your pretty little head about how they did it or how much of your disk space they used to do it.)
And darned if that hidden cache directory didn’t include full size image files of every site I’d visited during that time. Which stayed behind after I uninstalled the beta. How much data would there have been after a week of usage? A month?
Lame, Apple. Lame.