I don't like captchas. Most of the captchas around are simply to good nowadays - they cannot even be recognized by the human eye anymore. The little math questions are better but too easily overlocked.
Captcha!= scrambled image captcha. The maths questions are also forms of captcha, and personally I think their days are numbered because maths is too easy for a bot to solve.
I like the session solution, because it's not one I've come across before. The less prevalent a solution is, the longer it will be useful.