[I'm posting this here because although the experiences described relate to en.WP, I'm sure it applies to other projects as well]
I have trained over 100 people to edit Wikipedia this year; in around a dozen different sessions.
Not a single session has occurred, when someone has not had a problem with our CAPTCHA interface. Often, several editors in a single sesison are confused.
A user saves an edit, and the system responds with the requirement for them to complete a CAPTCHA .
Sometimes, they do not realise what has happened (the paragraphs beginning with the words "Your edit includes new external links..." seem insufficiently prominent), or they do not understand what is being asked of them (what words are they being asked to retype?), or they do not see the very small box where they are supposed to enter the CAPTCHA (on my screen, in Firefox and signed out, it is pre-populated with "Enter the words yc", where the "c" is half of a letter "o").
Naturally I am then able to assist them, but a user editing alone may simply abandon the task in frustration.
I urge each of you to try this, by editing while logged out
I suggest the WMF conduct some urgent usability tests around this feature, and either redesign or remove it.