To Mac or not to Mac

Our desktop computer is about four years old and starting to get pretty slow (which pretty much defeats the whole purpose of having a fast fiber optic connection. It's not urgent, but I'm at least starting to contemplate our next computer purchase, which raises the big question: Windows or Mac?

I've pretty much used Windows forever, and it's what I use at work, but everyone raves about the quality of Macs. Are they worth the extra money? Is it easy to switch back and forth between the two? Thoughts?