This is the first CAPTCHA replacement that I really like.

Adding a delay that costs computing power will both slow down and add real costs to spammers (though if it's a bot net they'll not care about the latter).

I really dislike how impossible CAPTCHAs have become, and the slew of startups working on advertising based ones is not the solution the users want.

As a user, all I want is ease. I'm fine waiting a few seconds... just don't give me an impossible task to perform or use the inconvenience you've created as an opportunity to advertise. All I want is to do whatever I wanted to do, read, write... I can wait a moment, just don't make it painful.