We switched to CircleCI from our own, painfully maintained Jenkins box to test our Rails apps. Between their clearly highly tuned infrastructure, nice little tweaks like caching the installed gemset between builds, and automatic parallelization of the test suite, our average build times dropped by 50-75% and became much more consistent. We did give up some flexibility in determining which branch names automatically run, or whether builds only run once a pull request is opened, but it's well worth it.