Take this rant with a grain of salt, as I am speaking it with tongue in cheek. To an extent. Generally speaking, I like Drupal 7. It's a very mature and flexible CMS. It also has lots of wise choices implemented. Overlay is not one of them. Overlay in Drupal 7 is a module, that loads admin pages in a modal overlay (hence the name) on top of your current content. The idea sounds sweet initially. You can make changes to your web site and you needn't leave the content page to make the changes. You can make all changes in an overlay, and then, when overlay closes, you will be back to your actual content pages, and no need to click it to the front page once the configuration has been done. Sounds sweet. Except it's not.
What's happening in a real Drupal world, is that as I am doing config, creating and deleting the content, making changes, clicking through yes and no of "Are you sure?" pages, I am having multiple page loads, as pages are reloading. Forms are submitted and evaluated. Caches cleared. Changes are taking effect. Now, what's happening to the background page? It's reloading as well. Two pages are actually being reloaded - one for the admin page in an overlay, and one on the background for content. Why do I need that, especially, if my front page on the background is heavy with views and has lot of content? I don't.
And this is where Overlay fails badly, at least for me. I understand why pages have to reload as forms validate, but I don't want them to reload twice and dong an 'extra step' in having to reload the background content. That is why, as soon as I am handed a Drupal 7 web site, I disable the Overlay in the first place (and I install Admin Menu second). Glad to see, that Overlay is gone in Drupal 8. Which makes Drupal 8 almost perfect.