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.