I've used both, and provided technical support for both. (Though not MUCH with Leopard yet - it's only been out a few days!)
The most important points have been covered, but to highlight:
-For most folks, at this point the user experience is pretty much the same.
-Mac hardware will run you more than PC hardware. I think Mac hardware is really solid, looks nice, and is well put together, but a comparably spec'd PC WILL be cheaper.
-If you're looking for computer gaming, a lot of games don't offer a Mac version. You can play World of Warcraft on the Mac, but not The Orange Box, to give a couple high profile examples. More things will NOT be available for Mac than will.
This extends somewhat to non-gaming software as well, but while the most popular games may NOT have a Mac version, the most popular other apps most likely WILL.
Something else to keep in mind with a Mac though is you can have your cake and eat it to (to an extent) through Boot Camp or Parallels - allowing you to run Windows apps from within Mac OS X, or just plain boot into Windows and run Windows on your Mac hardware. Note, of course, that Parallels isn't free, and you'd still have to buy (or obtain) a copy of the Windows OS to use as well.
Both Vista and Leopard do have some bumps right now - some application incompatibility, some areas where they're not quite stable. I know of a lot more for Vista, but that's probably at least partially due to it being out so much longer. Vista really needs a Service Pack update to get it over the hump and into the more stable OS realm. It doesn't suck, but it is in a similar place to where XP was when IT first came out... stability will come.