My daughter came home from school in tears after she discovered her iPhone 4 front glass had been shattered while in her backpack. She had always been very careful with her phone (really!).
Of course the phone still worked, but the glass fragments were becoming a hazard and the image from the front-facing camera was distorted.
Took about an hour and a half. Great instructions made it relatively easy.
The instructions on iFixit were really exceptional. We were thrilled when the phone worked perfectly on the first attempt to fix it!
The hardest part was making sure the ribbon cables attached to the digitizer did not get pinched and were of equal length before securing the digitizer to the aluminum frame.
Other than that, just the usual frustration dealing with really tiny fasteners was the only aggravation.
Take your time. Study the instructions carefully. Watch the video a couple of times before beginning. Use the tray template with the iFixit tray. It's invaluable for making sure no pieces get lost and everything is reassembled correctly.
Don't rush...You can do this.
Anti-Static Project Tray
iPhone 4 (GSM/AT&T) Screen
iFixit iPhone 4 and 4S Screen Protector