I'm currently using seven computers. Well, not at this precise moment (just three, as it happens), but darn it if I'm not proud of the fact. Of those seven, three run XP, one runs Ubuntu 6.06, two are now on Ubuntu 7.10, and one is Vista. Apple has invited me along to the Festival of the Leopard, so I have high hopes that I'll soon be adding OS X to the mix (I do have a Mac OS 8 box in the bedroom, but I only use that for Crystal Quest, so it doesn't count).
My XP systems, I like. Everything works with them, the one in the office lets me use the office Windows-only software (gnash) that controls the phones, and the two at home get loaded up with other bits of hardware and software that i can't be bothered to (or just can't) shoehorn into Linux.
My Ubuntu boxes, I love.