Kind of reminds me of my "Facebook plays Outpost 2" picture. Shame I can't find the topic, I don't think I still have the picture...
From a usefulness / security standpoint to my knowledge reCaptcha is pretty secure. They do adapt it to the failure rate of a certain client (i.e. if it becomes clear they are just guessing words and not attempting to solve the captcha it will add more perturbations to the graphics) and they do keep an eye out for IP addresses that are successfully solving many captchas in a short period of time (in other words, a computer program is attempting to solve them).I've thought about implementing recaptcha on the forums here to try and solve the spam problem (but I have a feeling that a lot of the spammers we get are being hand-registered).