Even though there are many changes listed, the ones that caught my eye in particular are:
KB 956531 - Outlook 2007 prompts you repeatedly for a password under certain network conditions
KB 956647 - Outlook 2007 does not honor the folder structure that was created in Internet Explorer 7 for the RSS Feeds folder
KB 956532 - When a user accesses their Free/Busy information, the Outlook Web Access service stops on the computer that is running Exchange 2007
KB 955572 - You cannot cache shared mail folders in Outlook 2007
KB 956528 - You cannot suppress the Autodiscover redirect warning in Outlook 2007
This hotfix fixes the following issues that were not previously documented in a Microsoft Knowledge Base article:
Outlook 2007 on a LAN uses NTLM authentication instead of Kerberos authentication for the Autodiscover service and for the Availability service.
You open Outlook 2007 in Cached Exchange Mode. After the initial synchronization is complete and the status bar displays All folders are up to date, the notification area still displays the synchronizing icon.
When you start Outlook 2007, you may receive the following error message: The data file file_name was not closed properly. The file is being checked for problems. Additionally, when the data consistency check occurs, Outlook 2007 responds slower than you expected. Note: This hotfix greatly reduces the frequency of the consistency checks and the time that is required to perform the checks. (Yay!)
Read more about the update here and request the binaries here. There are many more issues fixed than what I highlighted here.