Dropped my iPhone 3g on the pavement, a small crack in the corner. It didn't affect the touch screen, but sure was ugly.
The repair went smoothly, the instructions were great. It was difficult to separate the broken glass shards from the plastics, but it only took 15 minutes of careful work with a heat gun and the included spudger. The whole repair took about an hour, there was some difficulty with reconnecting the display cables.
Go slowly and take your time working on separating the front panel from the plastics. Make sure you route wires correctly and ensure they are plugged in right.