I realize that this is just a single data point, but I'm far more concerned that the App Store wouldn't instal Opera.
I asked that it install, confirmed that I was over 17, and watched as the App Store drew the Opera icon in my Dock with a loading thermometer across it. Then the Opera icon disappeared from the Dock, and I was miffed that Apple decided I didn't want the icon all that much. But it turned out that I didn't get to keep the icon because the App Store didn't install Opera.
So I went to the purchases page, and saw that Opera was listed, but that the Install button was active. (Which I see for other applications; those I have downloaded but since deleted.) I clicked the Install button, and tried to install Opera a second time. After a few moments, the App Store happily changed the Install button into an Installed icon.
Except that it's still not installed. Spotlight has no idea what the heck I'm talking about.
So I downloaded it from Opera's own website, and when I copied Opera.app to my Applications folder, it did so. It did not warn me about overwriting an existing application, which is hardly surprising, since the App Store clearly didn't actually do anything even after I asked it twice to install it.