Creamy smoothness?
Intel claims Hyper-Threading can offer real usability benefits of the same type we've always enjoyed from SMP. Here's the scenario: The user is running a host of different applications at once—web browser, e-mail client, instant messenger, MS Office apps, and something nasty from Adobe like Acrobat. The something nasty from Adobe conspires with a Flash app in the web browser and several MS Office apps to chew up all the user's CPU time, probably because something nasty from Adobe is in some kind of an unnecessary loop. The user's PC slows to a crawl, nearly unresponsive, because no CPU time is available.

The creamy smoothness of SMP is just this: no slowdown in that scenario. Intel claims similar things for Hyper-Threading.

Unfortunately, I haven't yet been able to decide whether and how much creamy smoothness Hyper-Threading is truly capable of delivering. Some of the benchmarks look promising, but making the subjective usability evaluation is more difficult. Truth be known, any 3GHz Pentium 4 system is so fast, it's tough to contrive the kinds of slow-downs that one can "feel" subjectively. I'll have to make a Hyper-Threaded PC my main system, full of MS Office apps, instant messaging programs, and nasty somethings from Adobe before I can make that call.

Conclusions
All in all, the Pentium 4 3.06GHz is the fastest PC processor available. The fastest Athlon XP system we tested, the 2800+ chip running on an nForce2 system, isn't widely available yet, over a month after we first reviewed it. The Athlon XP 2800+ can challenge even the Pentium 4 3.06GHz for supremacy; which one is faster depends entirely on what you want to do. You've seen the results, so you can decide for yourself which would best suit your needs.

Personally, I'd probably pick the Pentium 4 over the Athlon XP, simply because I'm all over fast busses, memory, and graphics. I also like to play the occasional video game when I'm not working on a review (once a year), and the P4 is just a teensy bit faster in most gaming tests. But it's very close.

That said, the significance of going from 2.8GHz to 3.06GHz is largely symbolic. And I don't even know exactly what it symbolizes. The steady march of progress in PC processors is, to me, much more impressive than reaching the occasional round-number milestone. But then I'm too close to this stuff, I suppose. "3GHz" is just easier to digest than "2.2GHz, 2.4GHz, 2.53GHz, 2.8GHz...."

Hyper-Threading technology, on the other hand, is truly novel. Our tests have shown performance increases in line with the "up to 30%" claims you may be hearing from Intel, provided the applications are multithreaded and the workloads are easily parallelizable. This is a real-world performance gain that's "free" for Pentium 4 buyers. Hyper-Threading's benefits aren't as universal as those of, say, a clock speed increase or a larger L2 cache, but they are compelling at times, especially if the user experience is truly enhanced by HT. At other times, Hyper-Threading is not a factor, and in rare situations, it causes slowdowns. As Intel's push for Hyper-Threading progresses, I expect to see more and more multithreaded and HT-aware apps arrive. We've seen this pattern before with SSE2 enhancements. Over time, Hyper-Threading's benefits should grow more pronounced.

Until then, if it causes problems, users can always turn Hyper-Threading off in the system BIOS, and they'll still have one of the fastest PC processors anywhere.