I've just uploaded the new questionnaire to my moodle 2.0 course and am about to play with it. This isn't an upgrade from 1.9 so I can't comment on any transition issues - so far the only things I have noticed have been theme related (for example the tabs "advanced settings/questions/preview" are not clickable in the Magazine theme I was using so I had to switch to a different one. I think that's the problem of the theme designers though, not the questionnaire writers (Only non-clickable when "no content" had been defined - when I switched to another theme and added some questions and switched back, the tabs in Magazine worked then.)
Well, so far so good - questionnaire worked, sent out emails, grades went to gradebook - exported into Excel and was able to re-use in another questionnaire -I haven't found these (deliberately!) hidden bugs yet, although I have noticed that students can't add images in the essay question but that is doubtless related to the tracker entry mentioned by Joseph above.
Thanks for testing, Mary. The bug you mention with the Magazine theme is really strange. I will investigate.