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The editing menus are not totally responsive. If you don't mind some swiping left and right, then you'll be fine. But no, it doesn't have a specific mobile editing UI.

Correct, mobile editing hasn't been a priority so far. Of course you can build mobile responsive sites well with it.

To me, JCR and Sling are superior to anything I've ever used in regard to content management. In my opinion, having such a powerful concept supports far more use cases than just serving content pages. Its a bit sad, that there are not more OSS projects on top of Sling or projects that try to implement similar concepts for different platforms (than Java).

Michelin uses it, there's a case study: https://apostrophecms.com/case-studies/michelin-case-study