Today I activated this plugin. When logged in to plugin dashword it gave a message "Your theme contains a meta description, which blocks WordPress SEO from working properly, please delete the following line, or press fix it:". I then clicked "fix it" button and it again asked me same question again. May be for some other code and I again clicked "fix it".
When I saw html view of my website home page and one inner page I found that the meta tags at the top had disappeared. I felt like this plugin is harming my site. Then I deactivated it but meta tags did not appear again. I tried changing the theme, deleting and reinstalling theme but no use. I thought I should completely uninstall WordPress.
After studying the plugin for sometime I tried to post new post and at the boottom of editor I found text boxes by SEO by Yoast plugin where WE CAN ADD META TAGS like meta keywords, meta description, title, etc. It also gives keyword suggestions. I felt this plugin looks good but what it has done to my meta tag.
Then I went to link http://www.quixapp.com/seo/ which you provided in you question and query my site and found meta tag available in my webpage. The I again opened the html view of my web page I found my webpage meta tag not on the top but little below. I found using find function in browser.
I felt very immotional when my meta tag disappeard but now I feel relief. I am trying to study the plugin completely. It looks good.