For the most part this is all fine with peoples' faces as there isn’t a convenient way to reverse the blur back into a photo so detailed that you can recognise the photo. So that’s good if that is what you intended. However, many people also resort to blurring sensitivenumbers andtext. I’ll illustrate why that is a BAD idea.
Suppose someone posted a photo of their check or credit card online for whatever awful reason (proving to Digg that I earned a million dollars, showing something funny about a check, comparing the size of something to a credit card, etc.), blurring out the image with the far-too-common mosaic effect to hide the numbers.