So do/can users get a preview of the content before being required to make payment? Or is the idea that users are likely already familiar with the site/content to which they will pay? Sorry I only skimmed the article.

The protocol itself only seems to define only the payment related negotiations. The author is free to implement some sort of preview themselves, though the full content should be on a separate page in order to use the browser's built in x401 client implementation.