I've done this on a couple well-visited client sites and it worked incredibly well. The best part is that it's super easy to add an invisible "email" field to a form and check if it has any content on the backend. By far the best bang for the buck.

I'm fairly sure I first learned this trick from Chris Shiflett's blog[1] a few years back, although I can't seem to find the original post.

1: http://shiflett.org/