Well...both run Office which is big for me work wise. OS X is an awful lot more stable than Windows - also updates of OS X dont screw up the use of every other program on your computer (like the recent XP update). The G4 unit in my iBook runs quicker than the P4 in my work computer.
Safari has it all over IE - it has some brilliantly simple ideas that I am amazed Microsoft couldn't come up with. Mail is a heap better than OE as well. iTunes is a brilliant music player - burn CD's by pressing 2 buttons! The whole iLife package is amazing. You can still get Windows Media Player for mac.
The only things I have had difficulty with is Messenger for Mac has sucked to use up to the last update, and now it is much more stable (I blame Microsoft, not Apple for that), and I was not able to use etax for my tax this year.
How has Apple lost the plot? They make much better computers than any of the windows operators, They have made two of the best computer inventions over the last few years in the iPod (will change the way people listen to music forever) and the Airport (particularly now Airport Express), and now with the G5 iMac, will change the way a desktop computer looks forever (if the others can figure out how they can do what Apple has with their gear).
Anybody who doesn't like Apples, are either biased, or haven't used one since the Apple II at school.