I'll confine my comments here to Word 2007, which aside from Outlook (which doesn't use the Ribbon in its main window) is the Office 2007 app I've had the chance to use most so far. I'm a big fan of using keyboard shortcuts to execute frequently-used commands in applications, so in my work with Word 2007 so far, I've tried to continue and use the keyboard shortcuts that I'm familiar with from Word 2003. However, although Office 2007 is advertised to support Office 2003 keyboard accelerator sequences, I have run across a couple of instances already where the Word 2003 keyboard accelerators are altered or broken in Word 2007.
Office 2007 also doesn't provide any "forward breadcrumbing" for Office 2003 keyboard accelerator sequences. For example, if I want to do the Office 2003 command "Table | Insert | Row Above", and I know that the 2003 accelerator sequence starts with Alt+a (Table), I (Insert), but then I'm not sure which letter of "Row Above" is the final step of the sequence ("r"? "a"? something else?) then I'm out of luck in Office 2007; Office 2007 shows the keystrokes of the sequence I've typed so far, but provides no feedback regarding the available keystrokes to continue the sequence.
Using a Word 2003 keyboard accelerator sequence in Word 2007
(In Office 2003, the menu would have opened along with my typing so that I'd be able to glance up and see the underlined letter in "Row Above" which is the correct final keystroke for my command. It turns out to be "a" in this case, in case you were wondering.)
So today, I tried to start using (and memorizing) the new keyboard accelerators built into Office 2007. 2007 does provide nice "forward breadcrumbing" for its new keyboard accelerators; when the Alt key is first pressed, the available shortcuts corresponding to each visible Ribbon option are displayed, and new available options continue to be displayed as you progress through the keystroke sequence.
My problem with the new 2007 accelerator sequences is that in general, they don't seem to lend themselves to memorization very well at all. In Office 2007, the accelerator sequence for "Insert Table Row Above" is alt+j, l, a. (Alt+j, l brings up the Layout ribbon section; a does the Insert Above.) There doesn't seem to be an "obvious" mnemonic for this sequence. (I found myself thinking "Justice League America!" for this particular sequence, but I'd hate to try and extend that paradigm to all 2007 accelerator sequences...)
Most 2003 sequences corresponded fairly well to the command you were executing, making them easy to remember. (Alt+e, f for Edit | Find, for example.) Even the 2003 sequence for Table | Insert | Row Above (alt+a, i, a) was pretty good when you remember that alt+a was the accelerator for the Table menu (alt+t being used for the Tools menu).
The "Insert Table Row Above" is actually not too bad as 2007 accelerator sequences go, either. The 2007 accelerator sequence for the basic "Find" operation is Alt+h, z, n, f, d, f -- 6 keystrokes! And not terribly intuitive, the most questionable sequence being "z, n" to open the "Editing" item on the "Home" ribbon.
The Word 2007 Ribbon after pressing Alt+h. "z, n" is the accelerator key sequence for the "Editing" item.
(You can, of course, alternatively use the Ctrl+F keyboard shortcut to bring up the Find dialog, but many commands don't have a simple Ctrl-key shortcut that can be used for quick access.)
Another point about Office 2007 keyboard accelerator sequences is that they will switch the active (visible) section of the Ribbon; so if you were working with the Home section of the Ribbon, and then type the 2007 "Insert Table Row Above" accelerator sequence, the Ribbon switches to make the Layout section visible, and you need to subsequently manually switch the ribbon back to the Home section (either by clicking it with the mouse or with the accelerator sequence alt+h, Esc, Esc) to make the Home icons visible once again.
My (early) overall impression is that the Office 2007 Ribbon is a nice step forward in UI design for novice/intermediate users (who will normally be activating commands by using the mouse, as opposed to the keyboard) and for the activation of seldom-used commands, but it is in some ways a step backwards for "power users" who prefer to activate frequently-used commands via the keyboard.