I can only speak from personal experience. I'm a profesisonal DTP/Word document specialist among several in the office where I work. We all found the switch challenging at first, and I spent significant time researching the keyboard shortcuts and recording some new macros to get my prodcutivity up to speed. (I work fast.) Once there, however, I have been generally pleased with the change, and think 2010 an improvement over 2007, though the difference is not funamental.

If you wish to actively master the new interface, you can probably do so in a few weeks; there are lots of resources on the web. Those who just pine for the old can buy 3rd-party tools to get back the old look -- but, remember that Office 2013 is now upon us, and Microsoft is, at least for now, commiting heavily to even newer types of tools.

Best,
Ulodesk