I've been using Ash for a few side projects and recently started using it at the day job too. We're mostly using it for new functionality, with the Ash APIs alongside our existing ones, but planning on slowly moving older things over too. It's been working well so far, and it's been easy enough to use the escape hatches for anything weird we're doing.

Getting started was a bit tricky though - definitely recommend the Ash book there. It works a lot better than the documentation as an introduction.