To me, the biggest advantage of Macs are ease of use.
I have worked with both Macs and PCs in my work environment(creative multimedia) and have been the most pleased with my Mac with OSX than I could ever be with Windows in any form. There are so many cool things OSX can do that only Vista can compete with, and we all know how long it'll be before that comes about!
I get great pleasure working with my Mac, and have learned to use all the keyboard shortcuts available. It's just so easy to do, and yet with Windows, it's not as easy, though you can do most of the same things. It's just better laid out on the keyboard. Plus, the interface is much nicer to me.
This is all very subjective. The point is not to say one is better than the other. The point is to say what works for you.
More to the point of the Mac ads, are they effective?
So far, Justin Long is not an asset to these ads. John Hodgman, however, is an essential part of the success of the ads. He's hilarious!!!