I have an Invisible Shield (front/back) on an iPad I'm borrowing from work. By far hardest install ever! Looks pretty good though, minimal "orange peel effect" and I know it's protected from scratches.
I'm getting a personal 3G iPad soon and have decided to go with Ghost Armor after seeing a cousin's iPad with Ghost Armor applied -- looks and feels much better than the Invisible Shield and is much more "invisible".
This is quite a well-covered territory topic though, so if you do happen to search, you might find even more tips/opinions. Power Support seems to be one of the most popular ones. Good luck, and don't try installing an Invisible Shield by yourself on an iPad -- not for the faint of heart. I know I won't ever do it again, even though I do a fantastic job on iPhone-sized gadgets.