For all projects that we plan to launch in the next couple of months we are using Drupal 6. We had to have the registration forms live with Captcha enabled! Drupal 6 Captcha module was in development. We decided we would use it and start addressing the issues as and when we faced them. We were pretty confident as there was a working Drupal 5 version for us to compare with.
Even while we tested it, we had some problems with the Captcha not functioning on form validate. The Captcha was not refreshing because of the form cache in Drupal 6. We have temporarily resolved by clearing the cache on every form load. This should not be the ideal way to approach the problem. We must actually refresh only the Captcha element and the rest of the form must be cached. How do we define a dynamic element within a form which doesn't get cached and gets refreshed??
My colleague Nagaraj helped us resolve this issue and identify the problem.
DO we really need Captcha, want to try: Safer forms!, a suggestion by Mr. Vasanth Govind.