Currently I use ...
Windows Vista - Main Desktop (Email, Office, Adobe Applications, and for Surfing the Web
Ubuntu Dapper Drake Dual Boot - For gaming both Linux games and Win Games using WINE.
To answer the original post by Invis.
Mac OSX Leopard promises to offer exciting new features and performance for the everyday user. Interoperability between Windows/Mac has gotten better in past years, but where there is non you can always take your old Windows XP license and install XP using Boot Camp on your Mac. User experience is always rated high on the Mac, and many people find that the Mac is much more stable. However, disregard any claims of Mac never crashing, and also disregard any claims that XP does any but crash. Both Operating Systems are built using the same hardware so issues come down to how much tinkering you do, and what kind of stress you put on your system.
Things to ask yourself when trying to decide between the two...
Is it in my budget?
Does it run all the applications I will need for school/work/play?
What is the cost of ownership?
Does the model have a good reliability rating?
What issues have other users had with OS/Mac Hardware?
How long will it take to learn?
Susceptibility to Virus's and Malware, and if moderate, how hard is it to remove said virus/malware?
What will repair costs come to if something breaks?
Is there a warranty?
Most people on this forum have tried to give a good comparison of -- others have expressed outright hatred for Mac/Win/Linux. I have used over 32 different Operating Systems for various reasons over the years ranging from Windows to Linux to Mac to Unix and many command line OS's. And it all comes down to purpose and need. Use a Mac if you believe it will be purposeful and fit your needs better than Windows. No one operating system is perfect and any who claims theirs is better is a self diluted egotistical never been laid idiot.
Finally, if you truly want to see if a Mac is right for you, go to a Mac store or CompUSA and try one out.