I spend a majority of my day on / near a computer due to my line of work... and I have some input on this.
MAC OS - Good, I like using it, but some of the things in it I really don't like. I am getting used to it, since I have only had my ibook for about a week and a half now.
Linux - Good, I also like it, but if you arn't willing to dedicate time to figuring out why things break... don't even bother running it. Things break all the time, configuration for things can be a pain at times. I've run slackware, redhat, mandrake, and most currently gentoo (stage 1 install).
Windows - Good, I like it for mindless computing. For everyone that complains... you have to admire what they have done. Imaging building a car with any engine, any powertrain, and size tires, and body and still having it work... yeah right. That will never happen. But with windows, it still manages to work even with all the hodge-podge hardware thats out there. Granted some things don't always go smoothly... but the sheer amount of hardware that windows supports that is almost 'plug and play' is absolutly amazing. Yes mac's run smoother, but apple also sells the hardware... so they can assure it works. Just imaging having to buy all your pc hardware from microsoft.
Just some food for thought. FYI, I have 3 computers sitting here right now. A new toshiba laptop running xp pro, a 2 year old ibook running the newest osX version and a desktop running gentoo linux... and I am posting from the windows machine because I am on lunch break from work and I want something mindless and easy.
BOT, I am downloading the beta right now so I can check it out. I ran an old beta of 'longhorn' many months ago and it was interesting.