I have always had 2 OSs running on my system, always split between a windows and linux based OS. Currently I have Win 7, and will stay here likely for a while because it is pretty decent. For the other half I have played with Mandriva, Mandrake, Red Hat linuxes (those are not free distributions), but I have landed on Ubuntu as a pretty decent free OS. Good for everything except gaming, I would say. Nice to have the versatility of 2 OSs. I haven't tried any of it's direct competitors, and I know a lot of people don't take Ubutntu seriously, but I say try it, it's extremely easy to set up and if you don't like it, don't keep it.
i find alot of people have this setup, even myself runs Windows XP and Linux Ubuntu on my PC, i even had Windows XP and Linux Mint dual boot on my netbook... now have downed my netbook to just one OS - Linux Mint due to space issues
i'm sure alot of geeky mac/apple pc owners have dual book with Windows or Linux... it's just handy having two OSs