We've spent a little bit of time checking out Safari 4 Beta on both Windows and Mac, and here's what we like (and don't like) so far:
On Windows, if you've never used Safari but used Chrome or Opera, it'll actually feel a lot like that, with the overhead tabs, Speed Dial, and even a few Chrome icons. After years of shunning standard Windows UI elements—which seemed especially dicky in a way, given how anal Apple can be about standardised UI themselves—Apple is actually playing nice. It's got the usual minimise, maximise and exit buttons, standard fonts and even real Windows shortcuts like Ctrl+Tab to cycle through tabs, so it actually mostly feels like it belongs on Windows now. Well, except for placing the X to kill tabs on the left side, which is annoying 'cause on Windows it should be on the right.
I'm still not quite sure how I feel about the pitch black look of the top tabs in Windows when the window is maximised if your default window colour is one of the darker shades—yeah, it matches, but I think Chrome's approach, with blue tabs set down a little bit that are easy to distinguish, works a little better. Overall, I think I prefer the tabs on top, at least in Windows. What I love for sure is that there's an actual arrow on the right side which you grab to tear off the tab into a new window—which results in a cool little pop-out animation, like it was a squished up sponge or something—because I'm always accidentally tearing off tabs in Firefox when I just wanted to move it down the line.
Top Sites is pretty slow to load the first time you fire up Safari 4, but then it's instant. You can't manually add sites—it's automagical—and once you delete 'em they're gone. A star pops up in the top right corner when there's something new at that page, which is a nice touch. Cover Flow moves smoothly, at first, but can get jerky if trying to move through a ton of sites at once. I hate how it replaces what ever site you're looking at if you just click the icon—you have to make a icon or middle-click so it does load over whatever you're lookin' at.
The smart search bar brings up suggestions from Google much faster than Firefox does, but the actual address bar is nowhere near as brilliant. It only pulls suggestions using the main part of the URL (before .com or .net or whatever), not from anything after a slash, or even the page's title. For instance, if you start typing "giz" and you've been to Gizmodo, it'll bring up Gizmodo. But typing Safari, even if you've been to Apple's Safari pages, won't bring up anything since it's not safari.com. As some have noted, the blue progress is gone, which is annoying, but I didn't notice it initially if that tells you anything.
Full search history seems like the crowning navigational jewel here. It actually searches the text of websites you've visited, and even with fifty or so pages in my history so far, it's really fast and works well. I just wish it made it more clear where on the page the text was located before you click, and how many instances there were, though otherwise the use of Cover Flow is nice.
It gobbles resources in Windows: With 14 tabs open, it ate over 400MB of RAM.
Overall, I think it's a lot more Windows-friendly than the initial Windows versions of Safari, and its excellent performance (if your system can stand up to its resource consumption) actually makes it worth taking a look at again if you didn't like it originally (I sure as hell didn't). It doesn't have the Mr. Fantastic-style extendability of Firefox and it's not quite as stripped down as Chrome, but it tries to balance between feature rich and clean, and doesn't do a bad job.
Despite its Windows improvements, the new UI does look much better on OS X overall—the new tabs on top look works really well, I think, though its break with a more standard OS X layout might vastly annoy some people. Also History and Cover Flow look and feel more natural on Mac too.
Another Mac perk: Multitouch zooming, which lets you manipulate the new full-page zoom with iPhone pinch gestures. It's not silky smooth, but definitely slick—it's actually a lot like browsing a page on the iPhone, especially if you're using two-finger scroll to pan and stuff.
Speed difference between Safari 4 and Firefox is a bit more noticeable in OS X. It doesn't kill Firefox, but there's definitely a difference. Cover Flow runs more smoothly, from what I'm seeing, without the jerkiness I got on Windows sometimes, though full history search isn't any quicker—but that's still plenty fast. It does use resources more efficiently in OS X than in Windows: Those same 14 tabs only ate about 230MB of memory.
While it's hard to speak to stability yet and whether or not you should use it as your main browser, it seems okay so far and the new features and UI make it worth grabbing, even if you wanna hang on to a more stable build of Safari or Firefox in the meantime.