When to use this moduile?
Since field types can be created and managed separately through both separate modules or the Custom Fields module, which should you choose? Which is a better fit for what you're trying to do?
What it's good for
The Custom Fields module provides a relatively simple interface for creating and managing your field types, so for things like making small change to the field types, adding new (simple) ones, adding validation rules, adding new settings and so on and so forth, it's perfect.
It's also exceedingly good as a workarea to try things out. The File Upload, Google Maps and WYSIWYG modules were all initially created via this module. I moved them into separate modules for a number of reasons, especially that they are optional. Placing them in separate modules allows users to only install those components that they need.
What it's not good for
Advanced stuff. The file upload field, for example, is placed in a separate module due to the complex server-side code that is needed to handle all the different places that files are uploaded. The Custom Fields interface also doesn't allow you to override code or template hooks: for that, you'll need a separate module.
If you require separate resource files (JS, etc), you can link to them via the module, but there's no facility to upload them to the site. So either you need to use FTP to upload them, or wrap them in a separate module.