The new Outlook WebApp shares a lot of the same UI features. So far, I've found it to be very functional and unobtrusive, so maybe the UI for other Office 2013 apps won't be as much of an issue as would at first seem to be the case.
Having said that, I only recently got fully up to speed with Office 2010 and have no desire to learn how to do what I already know how to do all over again. 2010 may end up being my last version of Office. I wouldn't be surprised if MS at some point chose to cripple it or make it unusable on Metro8, though.