SEOMoz has posted some original research on effect of CAPTCHAs on conversion rates:
With CAPTCHA’s on, SPAM and failed conversions accounted for 7.3% of all the conversions for the 3 month period. With CAPTCHA’s off, SPAM conversions accounted for 4.1% of all the conversions for the 3 month period. That possibly means when CAPTCHA’s are on, the company could lose out on 3.2% of all their conversions!
In other words, a significant proportion of frustrated customers simply abandon their attempts to get past the CAPTCHA. (And, notably, some spam still got through!)
We’ve blogged before about the usability problems of CAPTCHA-based forms, and it’s good to see some real-world data measuring those effects.
Akismet 
I abandon them all the time…I hate them, I can’t read them, I can’t be bothered with the audio versions…they are a very bad idea, and of course, they don’t work on a smartphone.
I avoid sites that use CAPTCHA like the plague. I’ve never seen one that didn’t fail to validate at least once if not several times. I honestly do not believe it actual works to stop spammers. I have very active blog and Akismet never fails to block spam. If anything I occasionally have to rescue legit comments.
I hate CAPTCHAs, especially those that require turning off NoScript.
I can see using them for major purposes, like when creating a blog or email account. Requiring them from commenters strikes me as a lazy unwillingness to use better methods of spam control.
I really hate CAPTCHA (=annoying thing)!
I avoid sites that use CAPTCHA because of all the problems. I’ve never seen one that didn’t fail to validate at least once if not several times in a row.
I love them, they keep idiots and morons out of my hair.
adam
I’ve abandoned a lot of Captchas, but almost always in commenting situations.
For registering for something I want (or need to pay for), I’ll almost always put up with them. That being said, the actual captcha technology matters too. The only one I can really handle easily is whatever the ST Captcha WordPress plugin uses. That’s easy enough I’m willing to use to protect registrations on a blog myself.
I generally have noticed that the only decent CAPTCHA is reCAPTCHA, and even it has it’s bad days where it fails to validate properly.