I switched last April, when I treated myself to the wonderful MBP, which was a birthday present to myself. The general slowness of Windows, crashes, freezes, and endless spam all contributed to my desire to change, plus my old Toshiba was showing its age.
My MBP is a splendid machine, beautiful, fast, powerful, and includes a terrific screen. In common with quite a number of others, I had thought about it for quite a while. My reasons for switching were, beautiful machines, gorgeous design and aesthetics; the iPod which I bought two years earlier, and iTunes (which knocks the socks off Macromedia), a superb design which works and is easy to use, as well as my experience of Apple customer support - (twice my iPod died within warranty and it was replaced twice without any difficulties or attempts to seek refuge in small print).
Other reasons included Apple's reputation for security - I had read about it on a BBC technology blog. When I had Windows, virus and spam attacks were frequent, and this despite the fact that I had purchased (rather than downloaded a free online version) a robust anti-spam, anti-virus package; while that reduced the daily deluge to a more limited number, unwelcome visitors were not entirely eliminated. With Apple, they have been almost completely eradicated (a tiny number still slip under the radar).
Of equal importance to me as a switcher, as someone who needs Word, who works with Word daily, and whose work life requires Word, (yes I accept that iWork is excellent, but Word is what is used in my work universe), is that Apple machines are now compatible with the software now used by a lot of the rest of the world, and one did not need to be a techie specialist to float between systems. I remember the difficulties of attempting to read or transfer documents from the two systems in the mid 1990s, and it was a bit of a bummer, to put it mildly. However, I'm really enjoying my MBP and would envisage remaining a Mac customer; the staff at the shop where I bought were helpfulness itself, and this forum has also been very welcoming and helpful. Cheers.