Choosing a CAPTCHA for a commercial product
Chris Heilmann has posted a detailed list of the pros and cons of several different methods for creating CAPTCHAs in CAPTCHA Alternatives for a commercial product?
All of the methods mentioned have both advantages and problems. Often the drawbacks are related to accessibility. The very nature of how CAPTCHAs work makes it difficult to come up with a single method that is fully accessible.
I think offering alternative CAPTCHAs would be best for both accessibility and usability. But exactly how that would work and which CAPTCHA alternatives to use, I don't know.