I’m not really an architecture/best practices guy, but here’s my take on your question. For entirely new components, the easiest thing to do is to package them in a SWC. In Flex Builder, you can do this by creating a Flex Library project and putting the components in there.

For customized versions of the built-in components, you have two options. If all you’re doing is skinning/styling them, then (as you probably know) you set up the skins and styles in a CSS stylesheet. If you’re additionally adding custom functionality, then you’d probably want to subclass them instead (or in addition), in which case you could package them in a Flex Library project as before.

Now, ideally, you could include both your CSS stylesheet (for skinned built-in components) and your custom components in the library SWC, and in fact Flex allows you to do this–if you call the stylesheet “defaults.css”, and put the SWC in the library path of your main app project, Flex will automatically link the stylesheet in. The problem is that Design View doesn’t currently support this.

So, my suggestion for now is:
– Put your custom components in a SWC
– Create a CSS stylesheet for all the skins/styles for built-in components as well as your custom components
– Hand these two things off to your users.

Your users will need to:
– Put the SWC in the library path of their Flex project
– Put the stylesheet in their main project source folder
– Add a tag to their main application

Hope this helps–let me know if it works for you.

Great work on Fl3!
I am interested in what the best practice approach is for Flex to have a set of re-usable UI building blocks, some of which might be skinned derivates of the build-in ones and some of which might be new components (compound or from scratch) for reuse within a group or department, all of which should look like they are visually branded in a certain way. What is the best way to create this and package it up for other Flex users to re-use? Thanks for any pointers!