I got tired of it in 2013. While it does work in some places (Android does it reasonably well), I haven't yet seen a good flat design on the desktop.

Windows 8 and 10 looked garish and hard to read, especially since everything is a rectangle with a one-pixel outline. Is it a button? Is it a text field? Maybe a thick progress bar? Who knows, they all look extremely similar.

While Apple did overdo it in the later big-cat OS X releases, I'll take a felt-textured widget panel and a calendar bound in leather over an endless sea of hairline rectangles.