[jtheory]

I don't like it for accessibility reasons, but if you have a site with n disabled users (say, a site for archers?) it might work well.

Certainly, not hard for spammers to break, but as long as it doesn't have widespread use, they won't bother putting in the effort.

[pornel]

> as it doesn't have widespread use, they won't bother putting in the effort

But this can be said about almost any Javascripty custom trick, and there are ones that no effort from user and don't have accessibility issues beyond requiring JS.