I like reCAPTCHA because I fail it about 30% of the time, even if what I type exactly matches the control word and the badly-scanned, blurry smudge next to it.
You know you've got a great CAPTCHA going when it keeps out all robots and a third of humans. And one that becomes progressively more difficult under certain circumstances (seriously, look at reCAPTCHA through a common Tor exit node. Shit's nuts).