I have been using Sitefinity for just 1 month and i found that it is a really nice CMS, especially with Module Builder, we can build custom content type very easily. But by using Module Builder to create new custom module, the CMS just made a few changes in some configuration files but not generating any code file. So i wonder how can we customize it for example, change the look and feel of back-end, front-end pages, create some complex data input check before publishing new content item, ...
Additionally you can check the Code reference generated by Sitefinity after you have created your custom module. In order to do that go to Administration->Module Builder->Your Module->Code reference for Your module in the top-right.
I believe you will also find our online resources useful and more specifically you can check the Blogs archive for more information. You can also explore Sitefinity SDK's Bug Tracker which is built using the Module builder.