We migrated all our CI/CD pipelines to CircleCI back in 2017 and are particularly happy about it since!
Our top loved feature is unlimited parallelism. We can run as many builds concurrently as we want.
We also use orbs, pipeline parameters, reusable commands, build cache, test insights.
For some of the heavier repos, we use a larger resource class and mount RAMdisk to a file system to speed up builds.