to be honest, as a user not a developer, i get hard times sometime distinguishing between the letter O and the number 0, or the letter I and the number 1 (or small L), this causes me to enter data manytimes, especially if there is password fields in the form, i usually end up filling it 2-3 times or ignoring the idea completely and canceling the request.
what is suggest is leaving the option to not include such misleading characters.
One more thing,
Another nice way of captcha, is instead of using noise (which i personally hate by the way), you can make it an equation.
for example, (write the sum of two and 3)
or, write the longest word among cat, lion, elephant.
write the tallest animal (ant, elephant, lion)
write the first two words only from I love telerik
and alike, of course all this should be in images to prevent Bots from solving it.
hope this helps.