Features built in to the os, such as ical (sorta) and software update...and stickies, and iphoto, actually get used. MS's built in stuff is so prone to virus attacks, and has such a big brother feel to it, that I don't want to even look at it. Seems like Apple wants to let you use the os the way you want to use it, and MS wants to tell you how to use their stuff.
As for PB specific stuff, I used to use it for takign notes, but I found professors don't really like it, and they tend to whine like little girls when you don't pander to their egos. Decided it was better to stay off the radar. It's reeeeally nice to take the thing to school, use wi-fi wherever, and be able to play games, manage software, look at web pages, and listen to music without going home, not to mention using ical and openoffice for schoolwork.