The No CAPTCHA problem
This is why I still recommend using other form spam prevention techniques before sacrificing usability for a CAPTCHA. One of the most effective combinations for 80%+ of the sites I've ever dealt with is having a honeypot field in the form, plus some amount of time required to pass before the form can be submitted successfully. There are other ways to mitigate bots as well, but these two alone have sufficed for quite some time for anything but dedicated human-based attacks.<p>Granted, I think the checkbox CAPTCHA is much better than the UX disaster that is the 'type some hard-to-read letters' CAPTCHA, but it's still adding a burden on the user, rather than a burden on the bot.<p>(Source: I maintain the Drupal Honeypot module[1], and have used it in a ton of different situations where CAPTCHA/reCAPTCHAs would normally be recommended).<p>[1] <a href="https://www.drupal.org/project/honeypot" rel="nofollow">https://www.drupal.org/project/honeypot</a>