I love this plugin as it is fun and much better looking than the standard captcha plugins.
However I'd like to report a clash with the WPMU Membership plugin. Upon clicking "Sign Up" to a particular subscription, the sign up form is called with no CSS or other site header, footer etc, and the message:
Fatal error: Call to a member function get_error_message() on a non-object in /home2/vitalit1/public_html/wp-content/plugins/sweetcaptcha-revolutionary-free-captcha-service/library/public.php on line 247
For now I will have to user another captcha but it would be great if this clash was resolved!