I am going to write an article soon about an actual Exchange 2003 to Exchange 2007 migration.
My first impression of Exchange 2007 - it still half baked in terms of GUI and some functionality. Not as bad as Exchange 2000 was, but still some things are missing. A lot of things can be done only via scripting or CLI (basically, a command prompt) which is really annoying and unlike Microsoft. I predict that a lot of these fatures will find themselves in GUI form when SP1 is released or via external tools.
More on this in my article when it is posted.