I thought maybe I was the only one who struggled with recaptcha captchas. On some sites I have literally tried 10-15 times before finally becoming too frustrated and giving up solving the captcha because whatever it was (form to fill out for a download, fill out a comment on a site), just wasn't worth the time I was wasting. Hugely irritating.
Initially, I was using reCAPTCHA on my site. But after hearing from irate users and specially preventing new users from registering, I decided to disable and use a simplified CAPTCHA. Loosing new potential users to CAPTCHA wasn't acceptable.