Various testimonials indicate a dislike for Microsoft's ribbon interface, which appears in SolidWorks, SpaceClaim, and AutoCAD. From my attempts at using it, I think the primary problem with the ribbon is that it creates confusion.
For a user interface to be effective, it has to be consistent. Consistant is not what the ribbon is about. It suffers from large buttons, small buttons. Buttons labelled with words, buttons labeled with icons. Sometimes what appears to be a button is only a text label; clicking it does nothing. At least AutoCAD's ribbon is customizable and turn-off-able; Microsoft was not sufficiently innovative to figure that out for the ribbon in its line of software.
(In the books I am updating for AutoCAD 2009, you'll see the ribbon in the illustrations. That's because it is part of AutoCAD's default user interface, not because I use it. Fortunately, you can return the familiar interface by switching to the "AutoCAD Legacy" workspace.)
One famous AutoCAD columnist tells me that he has reverted to using the sidescreen menu in AutoCAD 2009 -- AutoCAD's original user interface (other than the command line). It's still available: you can turn it on in the Options dialog box. He finds it faster.