My wife's iPhone 4 met an ugly end with some concrete. Front glass smashed. Front glass/panel/display unit replacement was the only option. Apple wanted $200, the replacement part on Amazon was $30....
iFixit's guide quoted a 60 minute 'hard' repair job. It took me about 3.5 hours -- but only because I was being super careful every step of the way. The phone worked first time after reassembly, much to my amazement.
Replacement parts are not all created equal. Some displays are non-retina resolution. If you don't mind an iPhone 4 with an iPod Touch 1st gen display, then go ahead and buy the $30 part. Spend more ($100?) for a true replacement part. Customer comments on whatever retail site will let you know what you're getting.
Use the printout sheet to track the loose screws from the iPhone. This was a huge help. There are more tiny little screws, all different, than the human brain can remember without help.
Magnetize your Philips 00. Don't attempt without the right tools!! Also, trust the reassembly to what feels right -- the iPhone was designed to be easy to put back together, think of the Foxconn assembly workers doing this day in day out.
It's not for the weak hearted but having an iPhone in tiny bits and being able to put it back together again means big bragging rights -- recommended if you have a steady hand.