I have been Microsoft free* for a couple of months now, as well, using Ubuntu GNU/Linux as my primary operating system, with OpenOffice.org as my office suite. I have not used Microsoft Office in almost a year.
Why do I prefer Ubuntu? Primarily, because it is lighter and faster, more reliable, and more configurable. It has encouraged me to focus on open standards and programs that work equally well on Windows and Mac OSX, so that I can move my data freely between all platforms. It also saves me money, especially on upgrades, although I do believe in contributing to the software projects from which I benefit.
I actually like Windows XP just fine, and always have. But my laptop has a 60GB hard drive, so Vista’s 20+GB install size was a dealbreaker for me. I think Microsoft really went the wrong direction with Vista, but I will not rule out a return to Windows if version 7 turns out to be lighter and faster, instead of bigger and slower.
*I do run Windows XP using VirtualBox (kind of like Parallels or VMWare Fusion for a Mac) so that I can use Adobe Acrobat, since there is still not a good Linux alternative. I also prefer the Windows-only scanner utility for my Fujitsu ScanSnap s1500, since Fujitsu still refuses to support Linux.