One of the nice features of iTunes/Address Book is the ability to Sync with services like Exchange, Yahoo! and Google in addition to Apple's own MobileMe. I decided to try these services at the risk of really screwing up my Address Book. I have about 700 entries in my address book. I use both groups and smart groups to organize my contacts. I use the notes, related people, and date fields in addition to the more pedestrian fields for name, address and phone number. Any sync scheme that I could use would need to support these features. I also use the picture field.