MadHouse raised a interesting point.
Sometimes I get so caught up as a Mac fanboy that I forget the real compelling argument for me:

I'm a big believer in UI development. I think design and UI is one of the most underrated parts of software and technology- this is coming from someone who is working on a major new development project for a fortune 15 company...and our UI sucks! Think about it like this: we eat with our eyes first. No one (hopefully) eyes a pile of canned dog food and says "ummmm that looks great" but look at something coming out of a the kitchen of a great restaurant and your mouth waters before you even smell it. UI is the same way. You could sit down in front of the must functional piece of software (why do you think so many industry specific apps are still terminal based ?...look at the airlines) but it its ugly as sin and hard to use then you've already lost the user acceptance battle.

Apple has done an amazing job at creating a highly attractive and very usable OS. When I see OS X I'm attracted to it so I want to use it, when I do use it, I work more effortlessly. One of the most underrated features of OS X is expose...I think if apple had invented a better way to access it (other than using the F keys) then more people would see how innovative it is. Another often begrudged feature is Spotlight, but I love it. Press Apple+space, type a few characters of anything (application, email, document contents...) and boom, it comes up. It almost eliveates the need to remember where you filed anything.

The tight integration of the hardware and the OS means that periferials usually 'just work. I'm no stranger to hacking drivers, digging into the windows registry, etc...but I dont want to do that. That's a waste of my time. I can plug my camera in, iPhoto loads the photos and I'm done. I can email them out easily, load them into flickr, publish them with iWeb, anything...and its quick and easy and I'm done.

From a geek point, I love the BSD underpinnings of OSX... I can drop to a terminal (usually have at least 3 open in the background) and do almost anything...and faster than I can on windows....but I'm a nerd like that. Again, its an example of a UI that works for me.

In the end, I guess what I'm saying is that the interface entices me to use it and when I do I find its easier to use. Don't confuse easier with a lack of power or control. I mean my workflow is cleaner, I can do tasks with less effort and in the end I get more done and enjoy doing it more.

That is one of the reasons I'm not compelled to run Vista in any major regard. It doesnt do enough to change how I use the computer. Frankly I'm dont have hopes that Lepoard (apple's next OS) will do that either. We are at a bit of an impasse in UI until someone figures out something totally different...I'm thinking hand gestures or something...but we'll just have to wait and see