This plugin is great if you just use it by itself. But, it does not work with categories or tags. I wanted to use this with the Facetious Search plugin. I was able to get it to sort of work with category search by changing Core.php to get the $category_id like this:
$category_id = get_category_by_slug($wp->query_vars["category_name"])->term_id;
The above mostly works. But, if you have posts in sub-categories that are not directly in the category you picked, it does not include them. In that way, it works differently than when you choose a category from the built-in wordpress widget. I didn't like that inconsistency in the interface. I couldn't see how to solve that myself.
Tags are completely not supported and I wish they were. I think they could be implemented exactly like categories are, but I didn't want to take that on myself.
So, I ended up not using this plugin, but I think it has great potential.