This is a variant on Schneier's fundamental theorem: Any fool can design a CAPTCHA for which they cannot program a solver. Just like in crypto, the only true test of CAPTCHAs are which one survive the test of time after having been attacked again and again, which is why it's very dangerous to jump on the bandwagon of a new CAPTCHA scheme (or worse yet -- design your own).

This is one reason why I'm partial to reCAPTCHA: there is a lot of experience in OCR systems, and we know what the current state of the art -- and we know what kind of things foil it.