Hmm. I just installed it, and after 5 minutes, I'm disappointed and unconvinced. The tabs now live in the window's title bar. Inactive tabs' close boxes only appear when you hover over the tab, a practice I generally despise (don't make me hunt for shit in the UI). The close boxes also look different from how they did in 3.x. The tab/title bar text doesn't look like title bar text (the font is different), giving Safari the fragile, unpolished feel of non-Mac-native apps (like Java apps or Qt-ported apps).
Most disappointing, Safari continues to SPOD when loading pages and there's any kind of I/O burden on the system (possibly even just the I/O burden caused by Safari itself). This is on a last-gen 15" MBP Core 2 Duo, 2.5 GHz, 4 GB RAM.
Going from a "top site" thumbnail to the actual page is slow, I'm sure due to typical Safari page rendering slowness.
I'm running Klicko, so it's hard to know how inactive window clicks actually behave, but there might be some oddness there. Overall, I'm not at all sure what the improvements are in this new Safari. Good thing it's a free update, but I think, so far, they should be paying me to update.