Ember has been a big win for us. Upgrading old apps is a breeze. Every app has a consistent structure and the conventions make it easy for developers to move between projects. We don't need to worry about the individual pieces - routing, data, etc - we trust the core devs so that we only worry about business logic. We're experimenting with Ember Engines right now and I'm not sure how we'd replicate it if we were using React or Angular. All in all, I think it has made us incredibly productive.
Outside of the code, the Ember team's communication skills are top notch. I feel like every time I tell my team that "XYZ is coming down the pipe", there's a clear plan of attack and any impact is minimal. In the last 3 years, I've never had to tell my boss that development will be paused to focus on a refactor/upgrade/maintenance.
The biggest downside is in hiring, unfortunately. I don't look for Ember experience, but I have had to adjust my expectations when it comes to the learning curve.