Has anyone ever attached a captcha to the survey submit?

LimeSurvey ships with a built-in captcha system that can be used for survey access, registration, save and load. It is fairly rudimentary and there is some concern that a more complicated service is needed (one with audio, for example).

1. Does anyone have experience with using an external service to provide captcha functionality in their surveys?
2. If you could populate a hidden field with the a flag that the captcha has been passed, I think you could use javascript to enable/disable the submit button (There are forum posts related to this).

Any thoughts and direction would be appreciated.
Best regards,
-Nebraska D