I've been using Windows since the early days (or if you want to include GUI-less OS's, I started with DOS).
I had kept my eye on OS X for a while, and I finally thought it was a viable alternative when Leopard came out. I wanted to try something different, and I was getting tired of XP, so I bought an Early 2008 MBP when they came out.
I've enjoyed it fairly well. However, I had to keep XP around for most of my audio/video production needs, so I dual-booted it.
Then the Windows 7 public beta came out. I started triple-booting. At that point I was using XP less and less, so by the time Windows 7 was finally released, I had pretty much stopped using XP.
As time went on I started using OS X (Snow Leopard at this point) less often. Eventually the only programs I was still running under OS X were Thunderbird and iTunes. I decided that was pointless, so I moved everything over to Windows 7.
I rarely boot into OS X anymore. There simply is no advantage to using it. Most of the software I use doesn't work on it. The dock is OK, but the Windows 7 taskbar is better.
Switcher is better than Expose. I could go on.
I don't think I'll abandon Apple completely, but I don't like where the company is going. Everything is just becoming too restrictive. I'll continue to use my full-sized Apple keyboard and my ancient iPod until they die, but at this point I have absolutely no interest in buying a new Mac.
===
As a side note, I used Windows Home Server v1 for a while (on my old P4 machine). It was the last remaining native installation of that 10-year-old, patched-to-hell-and-back NT kernel that I had running. It worked pretty well for file server use, especially with the drive pooling feature. Then one day a drive got some errors, and WHS threw up a BSOD. I rebooted and checked the drive with WD's standalone utility and it failed. Then I booted into WHS again, and it said there were no problems with any drives. BS. WHS, meet trash can.
I installed Debian on the P4, and I couldn't be happier. It's so much more reliable, more lightweight, faster, and more configurable than either Windows or OS X. On top of that it's completely free!