I first started working on this in 2009 using WordPress as a base with a CodeIgniter backend. In early 2009 it became necessary to completely rewrite it because it wasn’t very extensible and there were a lot of bugs and duplicate code.
By doing this, every update affects all the sites. In cases where custom functionality is needed for a client, we simply override with Kohana’s cascading file system.
I’ve learned more on this than on any other project before or after.
Please visit the link above for a full list of features.