While I haven't been wild about the AutoCAD 2009 Ribbon UI, I really like the way that they've implemented this for Inventor. Something about the way they organized it - and the way that you can still use the keyboard shortcuts works nicely. Also the way that you can easily drop into and out of sketch mode more transparently is nice. Perhaps it's because Inventor is fundamentally more structured in terms of contextual menus than AutoCAD (where you can really do any command at any time) - whereas in Inventor you're definitely in Part mode, Assembly mode and drawing mode (etc).
Why should we like Ribbons?
- Ribbons group tasks together logically
- Ribbons make obscure functions more "visible" - commands which are buried in menus and rarely see the light of day for regular users will be used more ("Discoverable", I think they call it).
Rather than doing like AutoCAD, where it was released in the 2009 version (a suprise for those who weren't involved in the beta program) - the Inventor team is releasing a "Technology Preview" in the middle of the 2009 version.
You can "safely" sample the functionality and provide feedback by downloading the "extended version" of Inventor, which runs happily side-by-side with regular Inventor.
While they've done some work to have 3rd Party add-ins start working automatically (depending on how you've setup your add-in) - I anticipate that there will still be some amount of trouble on that front. The challenge will be - will customers like it so much that they'll use it in production (in which case the add-ins will have to be updated NOW to support production, rather than waiting for Inventor 2010).
In any case - I'm impressed with how well it's implemented. They obviously put a lot of thought into it.