It works great on some websites, but not at all on other websites. Even though this is for the backend only, it may be the theme that causes it to not work on some sites. It also might be a plugin, though most of the plugins I have are on both a site where preview works, and on a site where it does not work.
Honestly, preview should be part of CF7 itself!
It is great when it works. Especially when developing a form that you do not want to include on your site yet, and do not want to create a dummy page just to display it.
No longer working. Now it just shows a blank background, where it used to show a preview. It has not been updated in quite some time. Perhaps it is not compatible with latest WP versions? It look like it may no longer be maintained? 🙁 CF7 itself should include a native previewer!