If the applications you need are available on the Mac, go for it. My desktop runs Linux while my laptop is an Apple G4 iBook w/12" screen (still stuck on XP at work). For my needs Max OS-X does everything I need.
The keyboard on my iBook is quite good and the screen is absolutely beautiful. Apple's mice are still only 1-button, with is a PITA IMO. Fortunately, OS-X supports two-button scroll wheel mice without any special configuration changes, so I bought a Logitech mouse rather than an Apple mouse to go along with my iBook.
On OS-X, I run Firefox for a browser, Thunderbird for email, and use OpenOffice.org for word processing and spreadsheets (MS Office is available for the Mac, though). I bought it in December and it's been rock solid.
Mac OS-X is absolutely stable and you don't need to worry about viruses and spyware. The UI is a bit different from Windows but it's quite easy to learn. And I love the fact that if I want to I can open up a terminal and be at a UNIX shell prompt.
The one upgrade I'm planning is to add more RAM. It came with 256 megs which is ok, but not great when I'm running a bunch of different apps. I'd set up a new Mac with a minimum of 512 megs of RAM; I'm going to upgrade my iBook to 768 total.
Even though I'm a long time Linux geek, I've given serious thought to switching to a Mac for my next desktop.
One thing you should do though is compare the cost of a 20" G5 iMac with that of a Mac Mini + keyboard, mouse, and 20" LCD. The iMac will have much better performance than the G4 Mini.