The biggest change in Office 2007 was the new Ribbon based UI. That Ribbon is loved by many and loathed by some. According to the data I saw from Microsoft the Office Ribbon has a high satisfaction factor among Office 2007 adopters.
What was the first thing that happened when Microsoft released Office 2007 with this strange new Ribbon? Well for one thing users floundered about trying to locate their favorite tools. The other major side effect was UI envy. Seems like a lot of companies wanted a Ribbon clone for their applications. This is where it gets tricky however. Microsoft requires you to sign a license if you use the Ribbon UI in your app. Even if you build your own Ribbon from scratch it is expected that you will sign this license. Read more on the troubles with the Ribbon license here.
New Ribbons APIs from Microsoft
This week at PDC 2008 Microsoft showed two new Ribbons. The first one, is part of the Windows 7 OS. I appears to based on a COM API. Since it is part of the OS it we should start to see more and more Ribbons appearing in application. Take a look at the WordPad and MSPaint screenshots.
The WPF team announced a release of a new Ribbon control. This Ribbon has a managed API and is customizable and skinnable. It is available on the CodePlex site.
There is a great article from Microsoft delineating guidelines for good Ribbon UI.