I've been using KMM on a commercial project with a team for a couple of sprints now (living the dream!) .
Going ok... I would say the biggest pain point so far is lack of direct Swift interop; which affects us in a couple of ways:
1) Inability to use Swift ABI 3rd party dependencies directly, but this can be worked around by injecting interface implementations down into common code.
2) Most important: we can't write SwiftUI code in the Kotlin Multiplatform shared module; this really limits the workflow we can use around building a separate UI component library to be shared between projects. This is something we'd really like to do.