Actually lately I've been reading a good number of people that are fed up with the new Mac OS and are actually going back to PC (I read several posts, yes I know this is not the majority of people and all machines are prone to problems). That's not something you hear of very often. In fact most Mac users are fiercely loyal to Apple and will beat you with a rusty pipe if you talk smack about their fancy cased PC (they are made out of all the same parts, it's just the OS that seperates them)
I've stuck with PCs because most times you get more bang for your buck, and if you are so inclined you can choose every single component. Also I do a lot of work in Maya, and at the time I purchased it, it was not available for the mac.
I almost switched last year, but once again their video car configs were either crazy expensive, or a generation old.
But design is design, has very little to do with the computer you use. Most software is available on both platforms so it's really just a matter of personal preference. There are a few things like Shake and Logic Audio that are Mac only (because Apple bought them) but there are similar tools that are available to the PC such as combustion, cubase, ProTools etc.
Oh and I'm running Vista 64 bit and it's been rock solid for me. I don't know why people say it's such crap, it just wasn't designed to run on older hardware, and most vendors are putting their priority in writing drivers for their new gear, not slaving over drivers for a 4 year old video card. It is a bit bloated though, and heavy on resources, but if you have a beefy machine it's a good operating system. I remember everyone saying how crappy XP was when it first came out, now people won't give it up. Same will happen with Vista once the service packs are released and it becomes more stable for more people.
Just my $.02