I tried the free version and it works great and delivers what it promises.
However I cannot use it because the plugin writes absolute path to the generated images.
I am running an Apache/php behind a Nginx reverse proxy server that add the SSL encryption for my entire company.
Since Apache is running http, the absolute path is also under http so I ended up with mixed content which was complained by most browsers.
I have a few http applications that got translated perfectly to https by Nginx and I have no ideas why it failed for this plugin.
However I see that the problem can be easily solved if you write relative paths to your generated images.
For free version, generated images are stored in media/xt-adaptive-images/...
Instead of writing http://mydomain/media/xt-adaptive-images..., writing /media/xt-adaptive-images/.... will perfectly solve my problem.