CAPTCHAs are often used to keep bots from sending Spam or opening mass accounts at Gmail or Yahoo! mail for example, yet they can be hard to solve and really annoying such as is the case with reCAPTCHA  which is helpful for digitizing scanned books and has a - difficult, too - audio-feature for visually impaired people.
CAPTCHA stands for Completely Automated Public Turing Test To Tell Computers and Humans Apart, and from the Turing test setup in the artificial intelligence domain we can draw the conclusion that if computers, or bots, can pass the CAPTCHA challenge-response test then the machines possess intelligence.
It wasn’t long after the first generation CAPTCHAs were introduced when CAPTCHA cracks were released, that is software which could read or interpret the distorted image. As such they passed the Turing test and showed intelligence. These days, reCAPTCHAs and similar technology cannot be solved by software, but the same is also true for many humans.
Moreover, CAPTCHAs are by no means helpful for deterring Spammers or mail account harvesters for example! While ‘unbreakable’ CAPTCHAs are a pain for legitimate users others buy a thousand human solved CAPTCHAs for a couple of dollars. If you don’t believe this check it out www.decaptcher.com or www.beatcaptchas.com which we found after some web research.
This clearly shows the case that CAPTCHAs are of little use against those who use bots which continue their work when the cheap labor country based human CAPTCHA operator earning less than two dollars for solving 1000 CAPTCHAs returns control to them. In contrast to cracking CAPTCHAs by software which is against the Digital Millennium Copyright Act (DMCA) human CAPTCHA solving by outsourcing/offshoring may well be legal.
As this article shows, CAPTCHAs have little effect against bots, and on the black market top brand email and other CAPTCHA protected digital accounts are sold in multiples of thousands accounts, so in my opinion its high time to introduce CAPTCHA alternatives.