I'm still on buildbot, but it's definitely showing its age and I'm hoping to move off of it within a year. I've been keeping an eye on Chromium's buildbot replacement, LUCI (https://ci.chromium.org/). It's still light on documentation and the source is very internal google-y (they seem to have written their own version of virtualenv in go). However, based on the design docs it does look like they ran into a lot of the same problems I have with buildbot, specifically the lack of support for dynamic workers, and how underpowered the buildbot build steps can be.
https://github.com/luci/recipes-py/blob/master/doc/user_guid...