Being dyslexic means I struggle so much with captchas that I often have to ask for help with them, which is embarrassing and so I usually end up not bothering (and the audio versions are very often no easier to deal with).
Good websites make the task of using them easier for their visitors, and don't make visitors bare the burden of administrative tasks.
Useful reading for site admins:
http://blogs.sitepoint.com/2007/09/1...-our-problems/