The most consistent user interface I've found is Open Office. I've used it on Windows, Linux, and the Mac and it's the same on all of them as much as it possibly can be. I'm amazed at how quickly I can be productive in OO on different platforms.
I find Office 2004 to be very different from any Office I've used, but, in fairness, I haven't used Office since the 97/2000 edition (which are almost identical), so the current Windows version could be different, too. I know when I was reading about Vista, Microsoft had changed Office so radically as to be unrecognizeable to people who had learned previous versions.
And that's ironic, because the original reason people didn't switch from Microsoft Office to the alternatives was usually that they had invested a lot of training time and money in Office. To suddenly migrate to Lotus SmartSuite (ok, bad example, it was preternaturally slow and crashed a lot) or something like that was out of the question. Now that there are viable competitors to MS Office, especially Open Office, it's unusual that MS would destroy the strength of their platform, namely the consistent interface. There's no real reason to upgrade anymore (the last Office version worth upgrading to was Office 97, which was the first stable 32-bit Windows version), but there is a huge reason to switch.
The last people I can think of who had an industry standard user interface that people were trained in and relied upon who dumped this interface was WordPerfect, whose ill-fated Windows version finished the company off. That version trashed a lot of the muscle-memory keystrokes people had learned. I would have figured MS to know about this (they're the ones who finished off WP, after all) and not make the same mistake of trashing their industry-standard interface.
I don't know if they work with end-users the way I used to. Even something like the change from Office 97 to 2000, with the most minor changes in how Word did certain things (I seem to remember lists being a problem), blew people's minds.
The compelling way to go is Open Office, where the user interface doesn't have to change for the sake of change as driven by marketing. I'm really surprised that MS didn't realize this and stick to their basic Office interface with Vista.
When using software, certain tasks become wired into muscle memory and don't require conscious thought after a while. This is one reason why software programmers use GNU Emacs, where the basic key bindings haven't changed since the 1970s. Companies who make office products need to get a clue about this and stop changing things. Go redesign the box, and the logo, and the color scheme, but don't change the fundamental way people do tasks.