We also use GO and Mambo/Joomla. Not sure if you know, but there is a thread on the old Mambo Forums about a bridge for Mambo/Joomla & GO...
http://forum.mamboserver.com/showthread ... oup+office
It works pretty well for us. One weak link, though, is not having a single logout. That is, if someone logs out of the Mambo/Joomla account, but doesn't logout of GO, they aren't necessarily logged out of GO. This really only becomes a problem with a shared workstation if the GO cookie hasn't expired by the time the next user logs into Joomla. If your users are not sharing computers than this may not be much of a problem for you.
Oh, something else to keep in mind: the bridge makes use of a wrapper within Joomla, so depending on your template you may have issues with side columns.