We have massively upgraded the address book features for all users. Some of the improved features include:
Vastly improved viewing, searching, sorting and editing interface
Many new fields (eg birthday, company, department, etc)
Multiple email, phone, fax and IM contacts
Multiple locations/mailing addresses
Multiple renameable custom fields
Improved uploading and merging facilities. More upload and download formats
These changes are all available immediately as a free upgrade to all accounts, and you can start using them just by clicking the Addresses link at the top of the page when you login to your account. If you need help with any of the new features, just use the Help link at the top of the page while on any of the address book screens to go to our FAQ. We'll continue to improve the FAQ based on questions we receive over the next few weeks.
Because the new address book has separate "First name(s)" and "Last name" fields while the old address book only had a "Full name" field, we've had to split existing address book entries. We've tried to do this intelligently, but please check over your address book to see that there's no problems.
We're also opening a forum thread available here so people can report any feedback and/or problems.