Just installed this, I'm on Mavericks (Mac OS 10.9.2), works just great! Entire screen (except for menubar) turns totally black, including even the desktop wallpaper. Very easy and simple to use.
You can change the color from black to another solid color of your choice, if you want. The color can be total, or it can let the actual background leak through, and you can decide the strength of that leakage, that's your choice too. You can also decide the fade speed. I left that at the original setting, at nearly immediate.
Whatever you want to focus on has to be selected first before you turn on Isolator. Isolator's icon appears only in the menubar. To turn on Isolator, you click its icon, then within its drop-down menu you click "on." When you're done, again click Isolator's icon, click "off" in its drop-down menu, then regular screen returns. That's it.
If you want to use the keyboard instead of the mouse, you can use the hotkey of your choice to turn Isolator on and off from the keyboard. The icon showing in the menubar is optional, you can still access it through the hotkey.
I see no bugs; it seems totally stable. This doesn't feel like it's a beta at all.
I did have two questions I couldn't resolve. In Preferences, under the tab titled "Background windows," there's something called "Filter type" where you have the choice of: None, Blur, Bloom, Crystals, Desaturate. I tried them all but couldn't determine what these options actually did. So I left this at the original setting, None. There's also something called "Filter radius," which you can change along a continuum from small to large. Tried this out as well, didn't seem to do anything either. I left this alone at the original setting as well. There's nothing on the developer's website that explains these two options either.