I'm less than impressed. This type of layout is something I've seen done before by everyone from Flipboard to paper.li, and I've never liked it on smaller screens. And as you can see from paper.li, this isn't something which is easy to do well.
What's more, I wonder if the Readability developers thought about how their users get the content. While some of Readability's competition picks the articles on behalf of the reader (Flipboard, for example), the average Readability user chooses to save the web content on their own.
This Grid View design is good for tempting readers into clicking an article link, but the average Readability user doesn't need to be tempted. They already decided to read the content they saved. I would argue that it would be better to stick with a list of saved articles. That is faster, more compact, and thus probably a better design for the smaller screen of the iPad Mini.