I just started using your contact form, and it's great -- lots of features, powerful, customizable, and yet easy to use. I gave it a five-star rating on WordPress.org. Thanks for all your hard work!
I had issues with the autoresponder at first, but then I discovered that the email service which was supposed to receive the autoresponder message was using clamav.net to filter out my my hosting provider InMotionHosting.com as a possible source of spam. If I send the autoresponder message to Gmail instead, it gets through.
I have a suggestion for others who are having autoresponder issues: If you use real From and Reply-To addresses in the autoresponder settings, you will at least get bounce messages notifying you that the recipient's email service thinks your site's autoresponse may be spam. That way, if there's a problem, at least you'll find out about it!