The service works surprisingly well. In the past we've used services that charge per CAPTCHA, which gets expensive. Switching to CAPTCHAs.IO has saved us quite a bit of money. I'm actually still a little baffled as to how they do it. It almost seems magical.
The one downside to their service is that the documentation is a little lacking. That said, once we were able to connect with them over Skype we were able to get all of our questions answered.