Google reCAPTCHA is a great idea originally developed at Carnegie Mellon University by Guatemalan computer scientist Luis von Ahn. It uses captcha to help digitize the text of books while protecting websites from bots. According to Google reports it displays over 100 million captchas every day. Among its subscribers are such popular sites as: Facebook, Twitter, CNN.com, and StumbleUpon.
So, main drawback is complexity of captchas. Captchas are getting more and more complex or even unreal to deal with. Just check twitter with query like “recaptcha” and you’ll see amazing amount of people that wonder what is going on.
number to think about
A number is 14%. According to my research on 2 our sites: http://prices.by and http://cartenergy.ru we were loosing about 14% of users on services sign-up while using REcaptcha.
The test was conducted using A/B testing where I vary our captcha and Google REcaptcha on sign-up page.
hint to google
Provide some parameter to select level of hardness, use of native languages is also a way to simplify solving while keep great security.