[wooster]

  The CAPTCHA's they use have been put through the ringer 
  and are proven to work against spammers who are focused 
  and resourceful.

Not really:

- http://securitylabs.websense.com/content/Blogs/2919.aspx

- http://www.cs.sfu.ca/~mori/research/gimpy/

- http://www.zdnet.co.uk/news/security-management/2004/05/06/s...

- http://caca.zoy.org/wiki/PWNtcha

etc. Most of those articles are from several years ago. The state of the art has improved since then.

[Gnolfo]

Sorry, I should have been clearer on what I meant. Google/etc CAPTCHAs don't work 100% against anti-CAPTCHA. As those articles suggest it's a fluctuating 80% or so, and obviously an ongoing arms race.

My point was more to the fact that the slider solution wouldn't retain anywhere near the same stats if it were put up against the same level of effort and sophistication spent towards breaking it. It would get solved and then bypassed completely.