My friend had cracked the back panel on her iPhone 4S last year and then last week, while waiting in line at Starbucks, a lady bumped into her and my friend dropped her iPhone again, this time cracking the display.
Apple would have charged $199 plus tax for a new iPhone, but iFixit charged $150 plus tax and shipping for the parts and tools. Plus, this way my friend didn't have to worry about making a backup and potentially losing data, since her iPhone wouldn't be replaced.
I got my friend's iPhone and the back panel replacement took all of 10 seconds. Replacing the display required taking virtually the entire iPhone apart. But thanks to iFixit's detailed guides, I was able to do it with no major issues. The front took about 90 minutes.
Be patient. The first time through, the front panel replacement can be tricky. If you've ever taken apart a laptop though, it's nothing too unfamiliar, just smaller.
Oh! And remember the home button! iFixit doesn't mention that in the guide, but the home button is a separate piece you'll need to move over to the new part.