What OS do you use and why?
I will start off this discussion...
I have three "functional" computers in my home right now. I have a large collection of non-functional computers but, since I don't use them, I will not talk about them.
1) My Wife's Computer: It is an Athlon 950 with about 384MB RAM. This machine runs Windows XP. It came with Windows ME when we bought it and we hated it! We upgraded it to XP as soon as it was possible.
Because this is my wife's machine I have no say on what OS she runs.
2) My Daughter's Computer: It is a Pentium 3(?) somewhere around 500mhz with 384MB of RAM We got this computer from a friend at work who was going to throw it away as he just bought a news one "'cause that old one got a virus or something..." The machine started out with Windows 98 but I upgraded it to Ubuntu a few months back.
3) My laptop/desktop Computer: It is a Celeron 1.2 with 256MB RAM. This machine was once a laptop but after the screen went bad it became a desktop. It still runs the original copy of XP it came with. It has been dual-booted a few times with different flavors of Linux but now it is just XP.
One reason we continue to use XP is iTunes. Though I have installed Rockbox onto my iPod that software is not mature enough for my iPod needs. Namely I can't plug in the car charger as it will drop from playing an mp3 to disk mode...
I also continue to use XP on my laptop as I write programs for windows users.
My wife will probably never switch away from XP as she uses her machine for all of our financial needs. Sad as it may be to say, there is no Quicken on Linux and some online banking sites still work funky with FireFox.