WordPress forums are full of unsympathetic moderators who tell you 'if you don't want your work stolen, don't post it up' type advice. They are not photographers. I am and I want to display my portfolio without watermarks.
Whilst we have to understand that there are always ways around image theft, there is nothing wrong with putting off 80% of the leechers, and this plugin does that well. More importantly, it doesn't disable all right-click features of a browser, so it won't annoy the innocent users of your website.
What I like is the ability to display a replacement image when a user right-clicks. This gives you the opportunity to use a copyright statement when they attempt to right-click.
One of the problems, however, is the user's ability to right-click to 'view source', where they'll find the direct link to the image which, when clicking on it, opens up in a new window, ready to be stolen. The plugin makes reference to this:
If you have uploaded your images to WordPress so that the images from the gallery can be opened in their own window, then this plugin will not work on the clicked image. Always upload images using FTP and insert the image via URL with no link to the image other than the IMG tag.
But that does not get round this problem. Whilst the average internet user probably doesn't use Inspector or Firebug, many will know how to check the source and find the image. If the plugin could circumvent this problem I would pay for a Pro version, but I appreciate this may never happen!