I'm an MCSE, supported and installed smaller and larger Exchange 5.5/2000/2003 Exchange server environments, and I got to say it...
I don't like where Exchange 2007 is going.
Microsoft products got to be where they are not because they are bug free and secure. They got there because of the friendly GUI, user and administrator expiriance and relative simplicity.
With Exchange 2007 those factors are going away, and the only reasons that corporations will consider to upgrade is the support lifecycle of Exchange 2003. I don't think that the new Exchange features are going to have such a large factors on the decission makers when it comes to spend a lot of money for new 64bit servers and new 64bit Windows server software and of course client access licenses.