The migration has been relatively smooth(probably better than going from XP to Vista), but the latest hiccup is the feature in iPhoto'08 to wall off your photos in Apple's iPhoto proprietary document repository that appears to be more locked down to a standard user than the iTunes library.
From what I can tell you can't easily get to individual pictures from the standard file system interfaces, so if you want to print some pictures to Costco via their web interface you have to first export from iPhoto the pictures you want into the Apple file system so the browser can see them. The older version of iPhoto would let you in the photo library from the file system but the new one does not. So there is now an extra step that is going to trip up basic users and if you are a new switcher, this will really be frustrating. On the PC it is pretty darn straight forward to print one's pictures online.
I hope there is a better work around available either by a suggestion from someone smarter than me or a new tool. The Costco website does mention a new Macintosh tool seems to be planned but nothing has been announced.
The search is on, Christy is not happy. ;-)
UPDATE (12/28/2007): There is a new version that appears to work with iPhoto'08.