Performance is only part of the story for a lot of people. I moved to a Mac a few months ago, not because of performance, but because of stability and ease of use.
I bought a 12"PB G4 1.33GHz with 1.25Gb which I now use at work in place of a new Dell 2.8GHz with 1Gb. Subjectively, the Dell felt a little snappier launching programs, but pretty marginal. My experience has been that Windows systems slow down over a few months use, as disk fragmentation sets in and software installs and upgrades bloat the registry. Don't even get me started on spyware!
After a few months with the PB, it has not slowed up at all, and I use it in a completely different fashion to the Dell system. With the Dell I would open and close applications all the time, because I know that windows does not multi-task particularly well, and stability suffers when you have a lot going on.
With the PB I just open whatever I want and leave it open while I'm working on a particular task. The PB doesn't seem to care how much you've got going on, it just carries on working. I can go to lunch and come back, wake up the PB and it's good to go - I'd never do this with a windows system without making sure everything was saved first.
Also subjectively, the PB feels quicker than my P4/2Gz/512Mb system running Xandros Linux.
So for me the stability counts for a lot. I haven't had to sacrifice performance with the PB, but if I had to I'd rather give up a little performance for a stable system than have to constantly be upgrading and cleaning the crap out of a Windows system.
Of course gamers will probably value performance higher than I do, but this is usually down to the video card as much as the CPU. It's all about horses for courses.