I expected that Visual Confirmation is good for bots and bad for humans ... But I didn't expect that numbers are so impressive.
Did you use the default Visual Confirmation, or Advanced Visual Confirmation?
I use the basic Visual Confirmation provided with phpBB. My 10% of humans fail statistic because they use lower case is skewed because I do not have a large enough sample to make the statistic meaningful. Plus the statistic was a guess. I went back and actually counted and here is what I got:
For the short period I had 11 "real user" registrations. I define a real user who has trouble as someone who posts no spam, has an IP address based near me (within 25 miles) and used lower case to initially respond to the Captcha.
7 (64%) Real Users had trouble or failed to register
2 (18%) Real Users failed to register due to Captcha difficulties
So basic captcha denies 18% of my potential real users and makes it hard or impossinle for 64% of my users.
I'm concerned about user experience, so when someone tells me to beef up my captcha I ... well... I look for alternatives. But with what I have now I have stopped all current spam attempts. I suspect my spam would still be zero if I threw away the visual confirmation, which would give me a better user experience.
Textual Confirmation re-displays the registation page, with an error message.
I'd not considered this option. Thanks. To tell the bot he failed the Visual Confirmation. I'm going to wait and see how the 404 error works, then I'll try this next.