I appreciate the work put into the core themes, and use them during core development and when teaching folks about WordPress. I've used 3 of them on production sites (and still use a couple of them), and recommend them to developers who are looking to get into theme development or for example themes to show potential clients.

I worked on one minor enhancement in Twenty Fifteen and from my experience, the work the theme designers and developers put into each one is high quality and well-thought-out, and is useful for learning and showcasing WordPress features.

I may not decide to use Twenty Sixteen in any of my sites, but you can be sure I'll read the code and learn some new things that I can apply to sites I build.