I switched about 5 or 6 years ago. To be honest, they are both just computers, so anything a PC can do a Mac can do. Depending on your Windows set up a Mac can be a faster & more secure environment, which is much harder to articulate, but you just seem to be able to get more done on a Mac than a windows machine.
I was using a friends Xp machine a few days ago, a modern dual core machine with loads of memory and it just struck me how old Xp is now and how much system overheads are consumed by antivirus and other start up programs. it just was not snappy at all. Never used Vista, so that may be much better.
A modern Mac is also able to run many OSes at native speed thanks to the intel transition, whilst you can do that on a windows box, you can't run Mac OS (legally anyway!) on non apple hardware.
I have just traded up to a MacPro, from a G5 iMac, in theory I could now run Windows apps in parallels or boot camp, thing is, all the tools I need exist on Mac OS.