I would say Mac OS X is the real benefit of running a Mac vs a PC:
It is rock stable I only managed to crash the system once in three years.
It has true multitasking you can do several tasks at the samt time.
OS X has protected memory and full support for multiprocessing.
The system is very modern, looks cool and has most drivers built in the system. If you, for instance, buy a new digital camera, no drivers will be needed, you just plug it in and go. Same thing with most printers, etc.
OS X also comes with some real good apps bundled, such as iTunes, QuickTime, iMovie, iPhoto, iDVD, iSync, iCal, Garageband etc. Not to mention simple things as the calculator; it is really nice and has real useful features such as one click update of currency exchange, built in speech, etc. Expose alone would make The Switch worthwhile.
There are currently no viruses for OS X. A virus protection company claimed last week they have found one, but it didn't qualify as a virus (shell script) as it needed installation of a physical person with your password. It is not likely to be widely spread. The news was more or less fabricated of this small company to scare us, making us buying their product.
Apple has made the transition from Mac OS 9 to OS X which are two completely different system. For most Macusers it has been amazingly simple. Microsoft and Windows are to make their step in a few years with Longhorn (the horns don't seem to be so long yet). Will it be without pain?
OS X is already in the world of 64-bit processing and can run 32-bit programs as well. Windows is not there yet.
If you need to run simple PC-programs on the Mac it is possible (but slow) with Virtual PC. Don't try to run games with advanced graphics though.
Apple is a innovative company which makes it real fun being a Macuser. They just never seem to run out of ideas of cool things to implement in the system or applications. Good Luck with your choice.