I just changed from a Wintel machine, which I've used for over 20 years, to a Mac. I had dragged my feet with Office 03 so long that people were starting to notice. I no longer could put off upgrading to the "new" Office interface.
Yes, I do not like the ribbon, but that really wasn't the problem. The real problem was that the changes Microsoft made to the Office Suite accomplished nothing and yet came at a high cost.
The new Office UI is very different but is not better. That is a complaint only old farts make (because they know the old ways), so Microsoft can just move ahead ignoring it. I wrestled with it for awhile, and then I figured, if I have to learn something new, why not learn Mac Keynote? I tried it, and found it was a modest improvement over PowerPoint, but that it didn't aggravate me so much because I no longer expected it to behave the same as the old version as I did with PowerPoint.
Pip Coburn, in The Change Function, says that users will change when the benefit of changing is greater than the perceived pain of making the change. That's the operative element here. There was no benefit and lots of pain. Microsoft didn't improve PowerPoint, they just moved the deck chairs around. That's pathetic and not the behavior of a market leader. FAIL.
Just for the record, I reject the argument that it is a zero-sum game between experienced and new users. That trade-off does exist, but only when physical manipulation is involved, such as twitch games, aircraft controls, and the like. Good UI is, in general, good for both experts and beginners alike.
I do not believe Microsoft's assertions that the ribbon is easy to learn. If you feed someone rotten fishheads for a while, then switch them over to a diet of fresh fishheads, they will be happier. You can then tout the statistical "fact" that "users prefer fresh fishheads," even though the truth is that they HATE fishheads. That, I believe, is how Microsoft gets its rationale for UI changes.