like nikon vs. canon, it all comes down to personal preference and what your uses are going to be. i know plenty of people that love macs and osx but still have to use windows because they have a class or their employer requires them to use a windows only app - this'll be perfect for them. as for running XP full time, i don't quite see that, but it's a definate possiblity that a percentage of people who buy a mac and slap XP on to dual boot will use windows a good amount of the time.
i personally stopped using windows over a year ago because i was getting too bogged down with having to update virus definitions every other day, scan my hard drive for ad- and mal-ware, go into regedit to fix some bonehead problem that was caused by god knows what, and reformatting my drive every 6 months because my system would just get slower and slower by the day. that and creative applications (adobe and the like) run a lot better on macs than they do on their PC counterparts - spare me any comparisons or benchmarks, those don't hold up in the real world when you have a bunch of other applications (AIM, web browser, FTP, itunes, word and excel, etc.) open at the same time.
just my .02