Things to Watch Out For

If you're targetting your site towards people with disablities, specifically visually impared users, then using a visual CAPTCHA is a bad idea since they will obviously have trouble entering the characters in the image.

Also, there are various ways to defeat CAPTCHA's, so they should only be considered one line in your defence against spam. Other tecniques can include blocking web site visiors by I.P. address or by the content in a form submission.