Summary and Conclusion
In reality, page load time is only a tiny part of the entire browser experience, and when Safari 4 is beating other browsers by just tenths of a second, that lead grows even less important. In all honesty, you won't ever notice that Firefox 3.5 loads pages a few milliseconds slower than Safari unless you set two machines beside each other and do a genuine A-B comparison. However, we do feel that some aspects of Firefox 3.5 would be missed if you shifted to Safari exclusively.
Another point is this: Top Sites may be nifty, but it's way too resource intensive. It brings netbooks to a crawl, and even on a modern MacBook Pro, the Top Sites page takes way too long to load. It's only useful if it's quick, and in its current form, it's not snappy enough. The MobileMe integration is great, but it's not free. Try convincing anyone that doesn't consider themselves a part of the "Mac faithful" that it's worth it, and you'll probably get more than one menacing scneer. Scoring 100/100 on the Acid 3 test is also a nice bragging right, but if you ever come across a site that just murders your browser of choice, you can simply fire up Safari for that one task and move on.
Really, we can't recommend Safari 4 as a primary browser simply due to the elegance of Firefox 3.5. While some plug-ins are available for Safari, the amount pales in comparison to the library available for Firefox. It's funny, really. This same kind of situation is what makes Apple's iPhone so hard to ignore. The dev community is so deep and the App Store is so loaded that every other smartphone just looks lackluster in comparison. The tables have turned on Apple in this situation, as Safari's (and Opera's, and Camino's, and IE 8's) third-party extras just can't measure up to those available for Firefox...yet, at least.
All told, Apple's Safari 4 is a solid and very fast browser. It's absolutely worth a download. We have a feeling that those fond of their Firefox plug-ins will have a hard time letting go, but if you've never been one to tinker with your browser's default settings, maybe Safari 4 is a better choice for you--especially on a Mac. It's leaps and bounds better than IE 8, and it's certainly more nimble than Camino and Opera. Over on the Windows side, Google's Chrome may have a good shot at outdoing Safari, but it's not quite there yet. Chrome is still a very new, and somewhat quirky, browser. It's not the fastest thing in the world, and it takes some getting used to (as do most Google applications). But, if you devote the time, you'll probably end up falling for Chrome, particularly if you rely heavily on Google Gears and Google Applications. As we've alluded to, browser choice is largely a personal preference, but we think Safari 4 still has aways to go before it leapfrogs the highly-supported Firefox, and it best keep innovating if it hopes to keep users who are considering the switch to Chrome.