Office Rocker Darren Strange (UK Product Manager for Office 2007) shares a great analogy to describe the experience of making the switch from Office 2003 to the new, ribbon-driven Office 2007. It's a lot like getting into a rental car at the airport and driving a particular model of car for the first time.
An excellent analogy offered to me by a smart analyst who has been using the beta was helpful. He compares adjusting to the new UI to the experience of driving a hire car out of an airport. You don't know the car you will get. Some things are in different places in the car, it feels a bit funny and the visibility is all different. You can take a while to get confident driving it even though you are a confident driver at home in your own car. The part where you get in and drive it slowly around the carpark, trying out the indicators and window wipers, accidentally tooting the horn and getting the feel of the brakes and gears. This is how it feels getting used to the new Office but soon you drive out of the car park and you are on your way. It doesn't take long and before you know it, you have adjusted. Sometimes when you get home again, your own car feels weird showing how you made the switch. This is how it feels if you use 2003 after using 2007 for a while. They call it the "sense of mastery" which is the feeling that the application is on your side and you know how it works.
I think that sums it up quite nicely.