If you're interested having this kind of setup for your own team, without having to muck with servers, you can use something like https://circleci.com. We use LXC, and pay lots of attention to having fast I/O. [disclosure: I'm founder/CTO of circle]
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.
at Codeship (https://codeship.io, I am one of the founders) we support Bitbucket and have been for a while. We use LXC as well and are currently looking into Docker for our next infrastructure steps.
I love github and have never used bitbucket or any other service, however I'm always disappointed that no one has tried creating a library that provides an abstraction layer for many of the features on github and other companies like github. This would help foster diversity in a market that is currently a monoculture.
