[Mac-zealot mode]Biggest benefit? No viruses, no product activation, no MS
OS X is a very nice OS to use for some. For others, XP is a nice OS to use. For others, Linux. To me, most of the ways of doing things on OS X just make sense, other things (such as ExposÃÃâ€*’©) are just 'cool'. As for graphics and audio work, while those things are very good on the Mac they're far from the only things it can do well.
Software-wise the iLife suite of applications (iTunes, iPhoto, iMovie, iDVD) are great for day-to-day use. iMovie and iDVD are a great way of creating your own DVDs if you have a digital camcorder (so I've heard - my 3 year old PowerBook has no DVD burner so doesn't come with iDVD). Safari (based on the Linux Konqueror browser) is a very promising browser. Mail.app is a very capable program, especially now that it is compatible with Exchange servers, and has a very good junk mail filter. iChat is a nice AIM-compatible chat program with audio and video chat capabilities.
Most of the software that people would need for day-to-day use is available (MS Office, Windows Media Player 9, RealOne, MS Remote Desktop Client).
OS X also has very good multi-language support (it even has a Welsh keyboard layout) and many apps have third-party localisations to support languages that they were not written for (although most of the included apps are localised in the 'major' languages - as an example, Safari is localised for Danish, Dutch, English, Finnish, French, German, Italian, Japanese, Korean, Norwegian, Spanish, Swedish, Chinese and Taiwanese).
It has built-in speech recognition (although your milage may vary with this one) and a large set of text-to-speech voices, a built-in spell-checker available to most applications (Safari is spell-checking this message as I type it) and comes with a good selection of development tools. It's also quite nice if you come from a UNIX background as it's based on a version of BSD UNIX.[/end Mac-zealot mode]
If you want to be able to play the latest games or if cost is at a premium (or if you're just more comfortable with Windows systems) then Windows is the way to go. The best thing would be to spend some time 'playing' with some of the Macs at your local PC World and see for yourself.
There's a (fairly) unbiased comparison between OS X and Windows XP (both the Home and Professional versions) at
http://www.xvsxp.com