I'm skeptical of a lot of promises Apple makes, but I download Safari 5 now and after playing with it, I'm pleasantly surprised. I'm on Snow Leopard and moved from Safari 4 to the Chrome betas and to the release version of Chrome when it came out. I prefer Safari's integration into the system, but Chrome's extensions and speed make it my primary browser (but when downloading PDFs, visiting Hulu, I'd have to go back to Safari). Safari 5 may make be switch back again, depending on the extension support.
Reader works pretty well; makes reading multiple part articles far more pleasant. Even on sites that don't break articles into multipage monsters (Ars Technica) the reader version was much better. Even better than that printing from the reader version prints the reader version (I'm doing a research project involving online newspaper articles and now I can simply print PDFs of the Reader version to Yojimbo!).
It seems a fair bit faster than Safari 4 or Chrome for OS X. There's been a few UI changes, especially in Top Sites/History, but overall it's the same beast.
The deciding factor is going to be extensions. I depend on Rikai-kun, Gmail Notifier, and Adblock for Chrome (plus I use flashblock, but there's already a good version for Safari, Click to Flash). If I can get those (and I wonder if/how Apple will deal with Adblock) I'll certainly move back to Safari; the fit and finish on it is much better than even the release version of Chrome; Chrome often doesn't shut down correctly, has crashed occasionally (and while page crashes aren't supposed to bring down the whole browser, they've make the browser unusable enough to warrant a restart), text input glitches, and webpages with semi-dynamic content (like a message window and then the subsequent message sent page) not visually updating for several seconds.
There's a second tier of extensions I'd like to see; Google Calendar, Amazon wishlist, etc.; hopefully Safari's extension community will be large enough that those also see the light of day.