Usually we are doing our best to maintain all of our controls, nevertheless in this case
I agree with you about the problems with the RadCaptcha and its documentation. They are both planned for improvement so that they can live up to the expectations.
As for the issue you encountered, there is a known problem with the Cache image storage location (enabled by default) of RadCaptcha, which could be causing the validation error. You can try setting the Session as a storage for the captcha image by following the first two steps from this help article
and checking if the problem persists. In case the captcha is still not working, could you please let me know if there is something specific in the setup of the control on your end that will allow me inspect the problem locally?
If you want to report similar problems or feature requests for improving the RadControls, do not hesitate to do so.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed