Most in depth I went was probably in http://m.youtube.com/watch?v=PUuYAksQWg4 but I discuss it in pretty much every talk I give.

You can also just ping me on Twitter and I can talk about it more. :)

Sean gave a (great) talk at RailsConf covering the attribute work as well as other Rails5 features "that you haven't heard of".

http://confreaks.tv/videos/railsconf2016-rails-5-features-yo...

I don't know if he has written about it beyond the docs and various commits. He has talked about it many times on his podcast (which I highly recommend). Here is an episode that talks about the attributes API as well as his Relation#or work: http://bikeshed.fm/8

Definitely also checkout the episodes where he talks about Diesel.