Of late recaptcha images have become excessively difficult to decipher. Is there any way to get analytics on unsuccessful attempts before a successful one for your website? I'm pretty certain the numbers are going to be bad and something needs to be done about it.
if validate_recap(params) #user validated the captcha, create an account else #user failed to validate, log the attempt and show a new captcha end
Using the reCAPTCHA library helps digitise books while reducing scripted signups, so it's worth sticking with it, but if you're worried that its affecting signups and want an alternative, you might consider using Mollom, Akismet, or Defensio, all of which attempt to grade form input to assess whether its human or not.