I think the point was that with the same specs, a computer running OS X would perform better than one with Windows (especially Vista). Also, in a lot of cases a new OS X will run faster on an older machine than the previous version (although now with so many things going on in the background, that's not always the case). With Windows releases, it's always the opposite.
Also, when Apple release a new machine, it's often the best in it's class (showed beautifully when Microsoft started to demo Vista and Office 2007 on iMacs and MacBook Pros), and in some cases you can't build your own PC to the same spec because a) the parts aren't on the market yet, and/or b) if you could get the parts, or the closest alternative, the cost would be higher than buying the Apple machine.
Although, yes; usually, you can build a better specced PC for less than a Mac.
Quite right; although again, I think this relates to Windows more than the hardware.
In my experience Windows goes wrong more often than OS X, and when it does, it's a lot harder to fix. Back when I used Windows almost exclusively, there'd usually be one day every week where something or everything just wouldn't go my way, and I'd spend a large part of the day getting it sorted before I could do what I want (but for the other 6 days of the week, it'd be fine). Now I'm using OS X pretty much exclusively, I've had 2 days like that in the past 18 months, and a reboot fixed everything first time.
I can't speak for Vista, as I've had no real experience with it; but I'd say even with XP I never really had a problem with the OS crashing (at least, no more than I have on OS X), but I could guarantee I'd be hitting the "End Now" button about 3 times a day minimum. It's actually very rare that I have to force quit an OS X app (and the ones I've had to do it most with are Word, and IE when I used them at college).