My opinion on Apple's use is mixed. In cases like the OS X lock screen, the use is great. It signifies an inability to continue until the device is unlocked while hinting at the content underneath.

Other uses are gratuitous and often the blurring and blending of colors creates weird sensations where what is underneath a window seems more important. I can see something is there, but can't make out the details, make me anxious that I'm missing something happening in the window.