Despite using the WP-reCAPTCHA plugin on all my sites, I've been seeing an increasing amount of comment spam on my clients' blogs. This is most likely due to human CAPTCHA solvers. When I switched to the Stop Spammer Registrations Plugin, it stopped comment spam completely, making reCAPTCHA obsolete.
Here's a tip for reCAPTCHA users: disable reCAPTCHA on blog comment forms. The reCAPTCHA box conflicts with the Stop Spammer Registrations Plugin's red herring form and causes an extra submit button to appear. Disabling reCAPTCHA on comment forms makes the extra submit button go away.
You don't need reCAPTCHA on comment forms anyway since the Stop Spammer Registrations Plugin is much more effective at stopping comment spam. You can still use reCAPTCHA on contact forms, if you want.