I recently started using kindle highlights along with https://readwise.io/ and it works like a charm!
Also, it allows me to export all of my highlights.
Finally it adds a gamification dimension, showing me random five hightlights from my books.
PS: Through this site I learned that my Kindle highlights reside in a in file called `My Clippings.txt` (in /Volumes/Kindle/documents/My Clippings.txt) This is something I always wanted to know, because I have many books I uploaded to my Kindle device via USB (so no clound sync available).
Unfortunately 'My Clippings.txt' is a narrowly (and honestly, pretty awfully) defined format that Kindle devices generate. So that won't work for Google Play Books.
That being said, we want to build a separate Google Play Books importing tool -- it's on our roadmap!