It was a build system written in C# and configured via XML-style job files. We moved because we felt that there were open source alternatives that were powerful

It was a build system written in C# and configured via XML-style job files. We moved because we felt that there were open source alternatives that were powerful, flexible, and would allow us to focus on more important things (ie the actual builds vs fiddling with features we may or may not have or fixing bugs in our proprietary system). There wasn't much point continuing to maintain this in-house anymore when existing tools could do everything we needed.