The rear glass panel was broken on an iPhone 4S I bought for my son. It just looks bad to have cracked phones and also increases the risk of water damage. The cracks will wick water into the phone. See capillary action.
So easy I didn't think I did it. Literally took 30 seconds. And with the 'Revelation Kit' I can see all the phones naughty bits inside!
Just do it. No one is impressed by a cracked case or screen.