I'm an AMD guy ever since the K6-2 (with 3DNow! Technology... ;))
I'm currently running an AthlonXP and have been through a few Athlons, a K6/K6-2 or two and I've been very happy with the speed and stability... the Intels I have owned however have not been so productive... from 486s to PII/PIIIs, I gave up on them... granted, I haven't tried a P4 yet but I made my choice - AMD all the way!
As for the Mac issue, I've used a few and owned a few (mostly older generation models) but I must say that I've been happy with their perfomance too... the operating systems on the whole are generally stable, the mouse having one button is actually ok when you're used to it... Windows spoilt us ;)
But I must also stand up for Macs here... I'm mainly a PC guy but they've taken quite a bashing from Intel, Microsoft and the whole Wintel crowd... shame really, they don't deserve that reputation!