We have installed a fresh build on Lync 2010 w/ Exchange 2010 integration. This is our second pass at it. We had an original installation which, while mostly working, had a few items that weren't correct from the start so we started over.
With that said - we have an odd issue where some of our test users are able to log on and send an IM out just fine (communication is both ways, etc) BUT when that person is added to a contact list and communication to them is initiated it fails (User not found, message not sent).
Their free/busy information is updating correctly within the Outlook client, etc. - you simply can't send an IM to them.