I have some users running ZCO and after they have upgraded to the 4.5.1 connector, we have a couple of problems.
1) The 'Outlook Today' page is gone/empty. It should be visible when clicking on the root of the 'Zimbra - User Name' folder. [Yes, a lot of people actually use this] If you right-click on the folder, go to Properties, then Home Page, the Address field is empty. It should read something like res://C:\Program Files\Microsoft Office\OFFICE11\1033\outlwvw.dll/outlook.htm. I tried pasting that in there and saving it, but it does not save the setting.
2) The 'Sharing' tab for folders is missing. It should be available on the various Calendars and Contacts lists for sharing with other users.
3) Some users are unable to show contact folders as Address Books. There should be an option in each Contacts folder to allow this, but it's greyed out. I was able to fix this for one person by uninstalling and reinstalling ZCO, but it hasn't worked for everyone.
These issues aren't happening to everyone, but enough that it doesn't seem to be isolated problems. They all seem to be related too, since the features are available in nearby places.