At it's heart, Windows 8/8.1 has to be considered from two perspectives.
The core is fine. It behaves much like Windows 7 and includes a few nice features like ISO mounting. It feels responsive and has great power management. Of course not all older applications play well with the changes to the core, as with any release of Windows. GOG tests the games for compatibility and a good number do support Windows 8.
The interface, however, is where a lot of disagreement comes into play. The Modern UI (formerly Metro) replaces the traditional smart screen. Instead of a list of applications in the corner of the screen you get a full screen display of the installed applications. There you can pin to start screen in categories or search and launch the application. There are also ad-supported Metro UI applications installed by default. You can use these as replacements for the widgets in Vista/7, but only on the start screen. Clicking on them will result in the Metro UI application opening full screen with a touch oriented interface. Note, the gestures work with the mouse as well.
If you choose not to use Modern UI or the start screen for any reason, there are a number of start menu replacements like Classic Shell, Start8, etc which can give you a Windows Vista/7 style menu and the option to boot to the desktop. This is what I do. I have set up Windows 8.1 with Classic Shell's menu and the option to boot to desktop. With that in place I think it's "close enough" to previous Windows releases to ease the transition to our new Modern UI overlords. :)
