This plugin works as advertised. I started a new blog and within 2-3 months I was getting rampant blog spam attempts. I run Bad Behavior plugin that blocks most attempts but some still get through, and the ones that do are really nasty porn spam.
I was deleting spam comments a dozen per day and it got really tiring.
Since installing Invisible Defender I have NOT GOT A SINGLE SPAM COMMENT! I can hardly believe my eyes.
The best part is, I had all my friends try leaving comments and they had no problem. I still seem to be getting the normal amount of real user comments, so I don't think there is any tradeoff with this plugin.
It's so functional I believe WordPress should have this built in.
The only drawback to this plugin is it's unprofessional hardcoded "Bye Bye, SPAMBOT!" response. I hacked the plugin PHP to say something like "Spam protection triggered. We really want to hear your comment, so if you feel this was a mistake, please contact us and let us know of the error."
Since obviously a spam bot will never read the original taunt, and only false positive humans instead, it makes a lot more sense to include a helpful message, than to taunt them when they are doing you the favor of leaving a blog comment...
I still give it a full 5 stars due to its brutal effectiveness and straight up simplicity! No JQuery or other bloat here. Just fast & easy spam protection.