Zuul is in a pilot phase for GoDaddy, primarily in use for testing their OpenStack deployment automation and also for managing changes to some of their Kubernetes deployment automation. They also uses Zuul to test and deploy itself.
GoDaddy has replaced a lot of slow, brittle Jenkins jobs with highly parallelized, more realistic Zuul test jobs. They've also been able to accelerate development on some new projects by setting up rapid proof-of-concepts in PRs to GitHub. Some of these go nowhere, and some of them eventually become the deployment automation that goes into production.
Zuul is currently being used as an offering within OpenLab for projects, applications and tools that need CI gating and/or automation around testing. With the companion tool Nodepool, they are able to keep OpenStack virtual machines available, speeding up the process for developers of testing code changes.
With the introduction of Zuul-based CI/CD instances, projects were able to use the cloud resources for their automation in a very dynamic way. As a result, projects can do much more excessive testing in less time, which directly results in higher quality software, while being faster in development and integration.
With the introduction of Zuul V3, OpenLab also sees a lot of benefits from the operator's perspective by providing a centrally hosted CI/CD instance, as opposed to many small ones that require managing individually.