I think that while it's great that there is the ability to change the displayed fields and the way the part renders through editing the xslt, I wish the Content Query web part would have implemented this in a similar manner as the Search Core Results web part. With the Search Core Results web part, you are able to specify the managed properties to retrieve and the xslt to use per instance of the web part. With the CQWP, the edits made to the ItemStyle.xslt applies globally to all instances of this web part so all instances selecting CustomStyle for the item style property will use the edited xslt, which isn't good if you're pulling different fields for each instance.