As a release engineer I've spent a lot of time with many different build systems from Cruise Control to Lunt Build ... and many, many others. Of all the systems I've used and tested buildbot was the one I loved. Instead of trying to be a Java build system for Ant/Maven where you can hack on older and new languages for building it allows you to script what you want done. This is a huge advantage when your building legacy code, Java code, and younger Python/Ruby code bases.
It also has a decent api to allow you to extend the functionality without spending lots of time trying to figure out how the developers implemented the system.
While it's Web UI does leave something to be desired it gets the job done.