iPhone 4 fell out of the belly pouch on my sweatshirt and promptly dropped on the hard garage floor. Sure enough rear glass was cracked.
Repair was extremely easy. I had a little issue at first getting the screwdriver tip to fit inside the screws. My eyes aren't as good as they used to be, and those screw heads ARE tiny. Overall, it was very straightforward. Took me about 5 minutes, and most of that time was me fumbling around trying to figure out the correct tip of the driver to use.
I wound up pulling the tip out of the screwdriver to initially get it to fit into the screwhead. I found that a bit easier for me, anyway. The new rear glass was an extremely tight fit relative to the old, broken glass. Maybe that was function of the old glass being broken.