Proof of work by itself is nearly useless, unfortunately. Compute is cheaper than people. This is one reason why CAPTCHA services will likely be with us always.

Proof of work by itself is nearly useless, unfortunately. Compute is cheaper than people. This is one reason why CAPTCHA services will likely be with us always. As someone working in the field, I also doubt your claim "will prevent 99.9% of spam" is based on real data. Modern headless browser spambots are not deterred by this kind of approach. (Edit: looks like the poster admitted this number was entirely made up later in the thread.)