I made the mistake of allowing iTunes to pull down the contacts from my gmail, which is where I manage all of my contacts' info, and Apple Address Book took it upon itself to add about 4 billion duplicates, create random contacts, and merge existing contacts. For instance, it merged my husband and his father, and created a random new contact with two email addresses: my Mother-In-Law's and my admin userename for my test domain... Um.... RANDOM! This then synced back to my phone and possibly all of my photo albums, which means they are all now ruined and will require retagging (who has time?!?!). Ironically, I did not allow Apple to pull down photos because it would force them through iPhoto, which I abandoned a few years ago due to major corruption and loss of photos. Now my photos are corrupted by Apple Address Book instead of Apple iPhoto.
Steve, you are fired.
I'm just glad I didn't chance allowing it to pull down my calendar stuff too.
probably the ickiest one is that Address Book merged my husband with his father. I'm sorry, but that's just creepy.