I was a Mac user from the late 80's until maybe 1995 or so. Then I got fed up with them, with OS issues, with cost, etc. So I left them behind for Windows. I stayed with Windows, even after I got the use of a 1999 PowerBook when someone at work left the company. OS 9 - which was what it came with - didn't particularly appeal to me. My Dell laptop had a better screen, bigger hard drive, etc. I'd ended up using Windows the way 99% of Windows users use it - merely as a way to get to their applications. I had a Dell PC at home that worked just fine for me and my family.
Then, in 2002, we moved and were in a temporary apartment without most of our stuff for a while - including the PC which had inadvertently been left in storage. We wanted to get a new PC anyway, as ours was almost four years old and 350 miles away. By chance, we walked into an Apple Store. We saw the 17" iMac and bought it that day - I wanted it to see what OS X was like, and my wife wanted it because it wasn't ugly. But my main goal was to get it home, set it up, and let my wife use it so she wouldn't bother me while I was working on my Dell laptop or call me while I was at work asking me how to do something.
I sat in front of that damn thing for virtually three straight days - Friday afternoon until I had to go to work (late) on Monday. It was the coolest thing I'd ever used. My wife was mad because she rarely got to use it until we bought the G5. I loved OS X Jaguar, and I love Panther. It took me only a few minutes to go from "I'll set this up so I can go back to my PC" to "why do I want to go back?".
The thing I've realized is that, while people use Windows to get to their apps - sort of like taking the car to get to the store - people often use Macs because of the OS itself in addition to their apps - sort of like taking the Jeep to the store just so you can go over through the woods to get there. OS X is fun. I don't know many people who love Windows. I know a lot of people who love what they can do with their Windows PC - meaning apps. I know a lot of people who love OS X in and of itself.
So be prepared to fall in love with it.
Practical advice: just play for a few days. Get one of the suggested books - I happen to think "Mac OS X Panther Killer Tips" by Scott Kelby is a fun one - one of those you can pick up and flip anywhere and see something cool.
OS X is stable but not perfect, so backup just like you'd backup any other system.
Don't worry about viruses, etc. Not yet, anyway.
Read this forum, post any questions you have, and realize that 95% of the time people will help you within the hour.