Scalafix user adoption is still quite low, scalafix-core got ~700 sonatype downloads in July. The last releases have had some sharp edges around the integrations (esp. sbt-scalafix, but also how to share/consume rewrites), which are partly addressed in the upcoming 0.5 release. I am continually testing it on larger projects, and plan to automate at some point the currently manual tests where I run scalafix on parts of the community build.
I am personally very excited to see cats v1.0 migration succeed!