This got a lot better in 1.4: TemplateResponse and the list filter API meant some gnarly code removal on my biggest customized admin project.
That said, I also chose to implement many of my admin changes at a higher level: templates which use jQuery, etc. and interact with the REST API for e.g. complex object creation are much easier than the crazy form / widget hacks I used to try.
That said, I also chose to implement many of my admin changes at a higher level: templates which use jQuery, etc. and interact with the REST API for e.g. complex object creation are much easier than the crazy form / widget hacks I used to try.