I just upgraded my wife's computer to Mac OS X 10.4. I was looking forward to being able to sharing our contacts in each of our address book. I haven't liked the .Mac synchronization because it makes a complete copy of the address book on both computers.
In Mac OS X 10.4, you are able to share you address book, and have other people subscribe to it. In addition to that, when you set up the shares, you can allow other people to modify the contacts. Here is a post on AppleInsider that describes this feature.
My wife didn't have a .Mac account, so I signed up for a email only account for her. I then set up my address book to share with her email address. When I when into her Address Book and attempted to subscribe to my address book, it said that she wasn't invited to subscribe. I then went to my computer and pushed the button to send the invitation. When I clicked on the link in her email, it put an entry in her Address Book.
Unfortunately, none of my contacts appeared in her Address Book. Maybe it is because she has an email only .Mac account. I figure that since she isn't storing any data on the servers, but would just be reading the data from mine, this would be fine. (Also there is nothing on the .Mac web site that says what kind of .Mac account will work with this.)
I was thinking that if I need to get a full .Mac account, I would see about getting a .Mac Family Pack. But I didn't see a way to upgrade a full account to a Family