Ok, I use them too, for commentary on my blog, to help keep from getting messages like this one...
I saw your posting and thought I would share the (job opportunity, penile enlargement system, prescription drug program, etc) that I found with you at www.stuffyoudon'twant.com
I find, however that half the time when I go to enter the letters shown to me I do so incorrectly. The l is leaning on the c and it could be a b or I get them jumbled in my head. How does this work to keep spammers at bay? Are spammers just twice as dumb as I am? I don't get it right half the time, so they don't ever get it right?
I wonder about this a lot.