1) When I first took a look at the Mac OS X address book, it didn't impress me much. After all, I already had an address book. Several in fact. But now that I have a BlueTooth capable cell phone, I'm beginning to appreciate the way all of the iApps work together seamlessly.
If your Mac doesn't have built-in Bluetooth (short range wireless) capability, you'll need a $50 gizmo. D-Link's DBT-120 works well. For more info on how to connect your Mac to your mobile phone, visit Apple's Knowledge Base Article 25419.
2) Simplicity and Power: The address book has a deceptively simple interface, but it's flexible enough for you to organize your contacts into groups and to customize your contact categories endlessly. Panther (OS X 10.3) has added some significant enhancements. Data is stored in vCard format. Use Eudora vCard Export to extract your address book from Eudora. To get your addresses back out of the Address Book, try this nifty utility.
3) Integration with iSync: The address book integrates with iSync so I can enter my contacts on the computer with mouse and keyboard instead of on my phone which, by the way, has a horrible user interface. Now if my phone gets lost or erased, I've got a backup. iSync also updated my Palm PDA's address book, so now I have the same list of contacts on all my digital devices, and no matter where I make changes, all get updated.
4) LDAP Functionality: I hate typing contacts into my address book. Who doesn't? But at work, we use an LDAP directory, so now I can search for my co-workers and simply drag their information (in vCard format) directly into my Address Book. Choose Address Book/Preferences... and choose the LDAP tab. Enter the name of your LDAP server (mine is ldap.nau.edu) and search base (ou=people, dc=nau, dc=edu). Now when you click on the "Directories" icon and type a name (or partial name) in the search box, you can look people up. If you find the person you were looking for, you can drag their contact information (in vCard format) into your Address Book. For more detailed directions, visit this page.
5) Integration with .Mac: Now my home computer and my work computer have the same collection of contacts because they have been synchronized via .Mac. The function for resolving duplicate entries nicely allows you to add, delete, merge or defer the decision. I couldn't ask for more.
6) Integration with Mail: When I get an e-mail from a friend or co-worker, I can just select Message/Add Sender to Address Book (Command-Y). Later, when creating a message, I can select the recipient from a list of individuals or groups in the Address Book. I can also e-mail an individual or group vCard to all my coworkers so they don't have to recreate all the entries. They can drop this vCard directly into their Address Book. I think it's finally time to say goodbye to my old friend, Eudora.
7) Integration with iCal: When I synced my Palm PDA with iSync, it uploaded my Appointments to iCal. Now I can make changes in iCal, on my Palm, or in another application that talks to the Palm (such as Oracle Calendar (aka Corporate Time), and those changes are applied everywhere when I sync up. I can also post my public calendars on the web with iCal.
8) Integration with iChatAV: When I'm working in the Address Book or Mail, the software indicates which of my contacts are currently online, and of course I can jump to the AIM compatible iChatAV and add any new people who chat with me directly to my address book and my "buddy list."