http://blog.codinghorror.com/captcha-effectiveness/ = written in 2006, but his points seem reasonable. And basic CAPTCHA tech has not changed much since 1997, so if 9 years later it was useless then why WOULD all the Big Names in tech still be using them for human verification before new account creation, comments, etc.
...but one of the comments pointed this out "Brain-n-brawns wrote in a day a captcha defeater."
http://www.brains-n-brawn.com/default.aspx?vDir=aicaptcha
This kind of topic discussion is what I love about the internet, and my actual hands-on understanding of some things that make it tick...
In the end... "hidden input field trick" FTW! https://www.webdigi.co.uk/blog/2009/does-your-website-really-need-a-captcha/