OCS is GREAT when you understand how it works. These two posts will save you loads of pain, and answers questions like: Why is my address book empty when I've JUST SIP enabled all users? Why do I get annoying authentication messages in Communicator, etc

Check these out

http://blogs.technet.com/uc/archive/2007/01/10/live-communications-server-2005-address-book-service-faq-troubleshooting.aspx

and

http://communicationsserverteam.com/archive/2007/12/17/52.aspx

When you're trying to figure out WHY, these will both help loads!

Thanks to the OCS team for great blogs