A little over a year ago, I moved my entire family to Mac computers. It was a tough pill to swallow spending all that money at once. I had a Mac Mini for several years but I never moved the rest of the family until spring 2008.
My daughter had thousands of viruses on her Windows pc. She didn't listen or care when I told her not to download games. All of our windows boxes had printer driver problems all the time. HP windows printer drivers are crap. Crap Crap Crap. But the same printers on OSX just plain work.
At this point, I have a policy that Windows is not allowed inside our firewall. Only OSX and Linux. My daughter starts at a prep school this fall and they require us to buy her a Windows box. Yuck. We will buy it, but it will be firewalled off so all she can do is use her vpn at school and print.
For me one big tradeoff is time. I don't get dragged downstairs on a daily basis to fix things that broke all by themselves. To me this is an important part of total cost of ownership. Yesterday I was sick. I spent almost the entire day in bed. My wife had an important document to type. Back in the day, this would require repeated interruptions to repair broken things on windows either for the network being "down" or printers that simply quit. She still asks about getting windows back and I just laugh.
There are some things in Windows I like better than OSX. For instance, the Finder is a joke compared to Explorer (not Internet Explorer mind you

). But there are a great many more things I like better in OSX than in Windows. One is Spotlight. Another is the fast start up and shutdown times both for standby and for rebooting. Another is the cpu utilization. My cpu is almost always sitting there like a devoted puppy ready to do what I want when I want it. It's not busy indexing for possible future searches or scanning for malware or trojans. Another thing that gets on my nerves is the modal dialog box. The modal dialog box should be the subject of a war crime tribunal at the Hague. It serves no purpose other than to render the machine unusable. I would love to get my hands on the windows kernel source code so I could comment out the whole section that makes it impossible to do anything else if a window has it's "modal" bit set. I would also make all dialogs time out. 90 seconds is all you ever need. OSX does some of these same annoying things, but it does it quarterly or perhaps semi annually, not once every 15 minutes.
Another reason I'm so down on windows is I have a windows machine for work. It is mal-administered by a group of people who could care less about how I spend my work day. It's all the same to them if my title were changed to "watcher of the hourglass". When I've had windows at home, I've been able to get all the security I needed with AVG free and Zonealarm. But since my daily exposure to windows is so painful, it's hard for me to be objective about how good windows
can be. Still, despite my best efforts, I cannot control crappy drivers. Crappy drivers are not an issue on OS X but they were my constant companion on windows.
There are some in this thread who claim the BSOD is a thing of the past. Oh come on already. I walked up to a Vista box in Office Depot to attempt to look something up. I wanted to make sure I was getting the correct toner for my printer. It bluescreened within seconds of my touching the mouse. I looked up the info on my Blackberry instead.
There is a definite advantage when you buy the hardware and the software from one company. And that is worth something. Is it worth the difference between the iMac and the homebuilt machine you are considering? Only you can answer that. For me the answer would be a resounding yes.