The customizer is one of the newest and most fast developing areas in WordPress. It’s also one of the tougher APIs to code for because there are a lot of moving parts and to get the most out of it you really need to be proficient with the javascript based API as well as the PHP based API. Here is an evolving list of resources that go beyond the Codex pages to help developers out as they build out their own customizer functionality.
Code Examples For Working With the Customizer API by the Theme Review Team
Various examples of working with the Customize API, such as creating simple and advanced panels/sections/controls/settings. But it’s mostly PHP and doesn’t touch much on the client side of things.
Code Example For Creating a Postmeta Based Customizer Setting
This is a working example of a postmeta based customizer setting by Weston Ruter. Note that some of this code won’t transfer seamlessly to, say a custom post type scenario without some adjustments, because it hardcodes for a post type of post.
The Complete Guide to the WordPress Theme Customizer
Daniel Pataki does a good job walking through a more complete tour of what you might be able to accomplish with JS driven logic.
Dependently-Contextual Customizer Controls
Weston Ruter guides you on creating Customizer controls that instantly toggle on or off depending on state using client-side logic. (see related gist)
Resetting the Customizer to a Blank Slate
A useful starting point if you want to create a dedicated customizer setup that is separate from the default experience.
Customizer tutorials and documentation
An older post by Justin Tadlock grouping some guides and articles on the customizer
And lastly two streams of resources of new guides and updates on the customizer are here:
XWP’s Make Blog Covering The Customizer
And the Make blog covering the customizer on WordPress.org: