Today at work I merged the pull request to internally enable Nixpkgs support for incremental Haskell builds at work, which led to a significant time savings for our CI builds (~50% reduction in time to merge, most of which is now the time to re-run tests)
The way we have it set up is that it automatically does a full build once a day (the first build after UTC midnight) and then every build after that is an incremental build that only rebuilds the diff since the last full rebuild.
I'm still waiting for upstream to (hopefully) merge and approve the relevant PRs, but in the meantime I'll write up a blog post explaining the motivation, design, and results of this work.