The CAPTCHA system of requiring people to input words or numbers from a distorted image seems to be an effective means of telling computers and humans apart, at least for the time being. At some point, computers will likely be able to be programmed to recognize distorted numbers and images as well, but for now it seems to work.
Yes, CAPTCHA is an effective way to tell humans and robots apart. However, as bots get more sophisticated, the new system of checking the box to confirm may now be as effective as the old system of reading and typing letters. That system took longer and was more annoying to deal with, but it was more secure.
No, CAPTCHA is not an effective security system to differentiate between humans and computers because as technology advances, programs and hackers can find ways to navigate around the cyber security system and get past the CAPTCHA system. CAPTCHA can be recognized by sophisticated programs and applications with advanced programming efforts.
The way in which some CAPTCHAs are presented makes it impossible for ordinary people to differentiate some of the words or numbers. This can be so frustrating for people that they just give up trying. This can result in orders not being placed or surveys not being submitted, so it doesn't really matter if it helps tell people and humans apart if it deters humans from completing the task they were trying to accomplish.