The preview will show you exactly how your post will look before you publish it, and it is fast and convenient. It happens right inside the composer, when you click on the Preview button a dialog pops up full screen to show you the post as if it's published. The URL of the post is printed on top of the preview window. To exit you can press Esc or hit Close button.
One thing I want to point out, by design I want each Fanray theme to provide a
content.css. This file is meant for the editor to consume, it includes things like the typography and the content width styles of your particular theme. It enables the users to see a similar styling to the final published post while they are still editing.
content.css does not make your content 100% resemble the eventual published post. This is due to some of the limitations of the editor, for example it currently does not support block source code which is a known issue and will be addressed in the future. In the meantime, I have come up with a shortcode for you to input block source code into your post.
The ability to preview a post before publish is a much needed feature for the increased usability of the blog composer. It'd be even nicer if it could have a keyboard shortcut assigned to the preview to help user get into the preview quickly. Also if you are writing a long post it'd be helpful if there is an option to sync the preview and the editor so you don't always start from the top and have to scroll down. These nice-to-have additions I don't have time to pursue right now, if anyone in the community is interested please feel free to improve on it.
I hope you find this feature useful :)