I've been using Office 2007 through a couple of beta versions, and now for real and despite some early hiccups I'm hooked. At home on Storms machine I still have to revert to Office 2003 and I find it really disorientating.
Outlook is probably the apps that's had the most obvious changes in ramping up the smarts that I use every day but the ribbon and smart mini-toolbars in Word really are intuitive and make it much easier to find functionality that was often hidden behind layers of menus. The great thing is the old keystrokes you knew and loved still work so keyboard power users won't be thrown off their stroke.
While it's not the productivity app of my dreams I did enjoy flipping through The Enchanted Office (or "Once Upon A User Interface") today.
I don't think I'm going to dream about productivity gains tonight, but I might just roll Office 12 out to Storms machine...