I just read Robin Capper's 2007: A ADT/ABS Space Odyssey that describes the new unified space object. This is one of the major reasons why you should upgrade to ADT 2007. To be able to create spaces for all rooms in a drawing within seconds and to have the spaces associated with the bounding walls and objects is a real time saver. Visual Styles in AutoCAD/Architectural Desktop includes a video showing what Visual Styles is about.
Matt Dillon also has a fair post about his Position on "Revit vs. ADT" or was it ADT vs. Revit. It's not specific about ADT 2007 but worth reading anyway. I agree that ADT is more than only AutoCAD for Architects.
If you want to know about the great Enhanced Property Data Features in ADT 2007 David Koch has posted 5 parts about it. Let's see if more is coming about it.
I have also a post on the Scheduling Enhancements in ADT 2007. And as the last tip the Palette Auto-hide Speed freeware is working great in AutoCAD 2007 and ADT 2007 as well.
Do I recommend upgrading to ADT 2007? Sure I do. But as always it's really up to how you use ADT and your environment and other factors. Remember that it's a new DWG format so you will not be able to use the ADT objects once saved in 2007 in older versions.