After only having my new iPad 4 a week my 4 year old decided he wanted to try even though he knew it was hands-off :) The iPad slipped from his hands and the corner hit the concrete shattering the screen. It was a drop of less than 2' from my TV stand to the floor, but it hit just the right spot. Luckily, the Zagg shield held the glass in place so it didn't do any damage to the LCD.
I thoroughly read the manuals here for the iPad 3 and checked several videos online before starting. For the most part, the repair went well. The glass separated from the iPad where it was whole, but in ran into some trouble around the shattered areas.
The glass had to be pulled from the device and the broken pieces had to be carefully pried away later. The glue is very strong and makes it quite difficult to get small pieces off easily. Keeping the glue warm is absolutely key! I had to use a precision flat-head screwdriver to pry the small parts and remaining glue off of the iPad (a step omitted from the guide).
The corners of the case needed to be bent a bit for the new glass to fit (they were damaged in the drop). Overall I'm happy with the results and saved a ton of money doing it myself.
The iPad 4 used the same glass and digitizer as the iPad 3. The only difference is how the home button attaches. On the iPad 4, there is a ribbon cable attached to the button which requires you to hold the new glass at a 45 degree angle to the iPad before you can reattach it.
Oh, and get a good case with little kids around (OtterBox or LifeProof). These things are FRAGILE!