Agree with all of the reasons cited in the posts above; beautiful aesthetic, well-built, it works immediately, extremely good design (compare Macromedia which is ghastly with iTunes), excellent security (very little spam, no viruses), mostly very good customer support (two iPods of mine died within warranty and were replaced without fuss), and these days, with software such as Office for Mac, Apples are accessible to those obliged to work from/with other software which was not the case some years ago.
Anyway, I'm a recent switcher and very happy with my MBP. I'd cite an additional reason. Customer satisfaction; you can read the comments to people contemplating switching, those who have switched and those who did so years ago. How many want to go back to the world of Windows? Word of mouth, or of pen, is far more powerful than all of the advertising and spin as a recommendation.
Personalities are a different matter again. I would argue that Bill Gates is quite an attractive human being in some ways; what he plans to do with his personal foundation is very impressive and demonstrates a social conscience and degree of economic and political awareness that are very praiseworthy. However, as a designer, his products do not match those of Apple.
Cheers.