Not that smooth if you saw the comments on HN last week - teams lost months of productivity keeping up with the rewrites in Ember, even with no immediate breaking changes.
Angular is my 'plan b' while I continue working with ExtJS.
Last December I coded up just enough of an app in Angular to ensure that my Sencha-focused Flask app would work with another JS framework should the need ever arise. I'd hate to have to go that route of course (even if Sencha changes hands), but it's nice to have options.