The BIG downside to "captcha"-like technologies is that it limits access to those who can use image-oriented interfaces. That excludes the whole blind population, or those who use text-only browsers for other reasons. (That also means the US Government can't buy or use your methods, since they can't legally exclude a disabled population so cavalierly.) Before you dismiss that group out of whim, ask yourself if there is any other way to accomplish your goals.

For every perceived problem, there is a solution that is obvious, easy, and wrong.