I bought a Mac after 10+ years of building/tweaking PC's, and I did so for a few good reasons.
I wanted something different. I bought an iPod in Feb. 06 and I loved it, the simplicity, the integration into iTunes, etc. I wanted a computer that worked as well as the iPod. For me the Mac is almost a minimalist machine, although it is very robust. OSX isn't nearly the resource hog that Windows is. 500mHz G3's are running OSX 10.4.x without much issue. Try running Vista on a PIII 500mHz.
I didn't want to have to upgrade so frequently to do the latest thing. My PC is nearing the end of its useful life-cycle at the 2.5 year mark, and the only reason I would upgrade is to play newer games. Honestly, with the advances in consoles (read: PS3) gaming on a PC is not a necessity for me anymore. There are a few titles that I play on PC, and all of those run great on my current machine, leaving no reason to upgrade in that regard.
Windows Vista. After using release candidates of Vista and reading about all the "big brother" features, I decided I didn't want anything to do with it. Not that I have anything to hide, I just don't want my every move tracked by Microsoft.
OSX. Beautiful, simple, clean, fast, very few bugs. Although I haven't had many issues with my XP box, I wanted to be rid of the 10x per week Windows updates, virus updates, etc. that plague the Windows platform. I've had my Mac Mini for a month now, and I've had one update since the initial update from 10.4.6 to 10.4.8. No viruses, no spyware, no malware, no disk defragmenting, none of that.
Everything that I use a computer for says use a Mac. I do a lot of work in Photoshop, probably 3+ hours per day. I also do a lot of digital photography, so much so that I invested in a 10.2mpx DSLR (Sony Alpha-100) camera. I do a bit of audio recording as well, and have a reasonable music library in iTunes. I know the PC does all of this, but the Mac versions of most of these programs are better, mainly because that is their native version.
Anyway, after 10+ years of being a power user on the PC, I'm finding myself rolling in simplicity in the new Mac, absolutely loving (almost) every minute of it. I do use my XP box still, but mainly it sits as a file server and I use it to rip DVDs, because nothing on the Mac rips anywhere near as fast as a PC.