The iPad screen is glass - it doesn't scratch easily.
I really don't think you need a screen protector. I've been using mine with nothing on it for weeks and not a single scratch or imperfection on it. I do have the Apple case on it to protect it from accidents of course.
I don't think I'd want to use it. After putting it on my LG Voyager cell phone successfuly (Verizon doesn't do iPhones yet), I bought the whole shield kit for my HP 17.3" laptop and it was really difficult to put on and it doesn't look as good as the one I did on my cell phone. It was okay on some of the smaller spots but when it came to putting the larger pieces on, it was bad news and it wasn't an exact fit like it should have been. Plus you have to use water with it and that's kind of a risk too. I bought a case for the iPad that's supposed to be delivered to me but wasn't. The case is coming with three screen protectors. I'm not sure how good they'll be either. They are supposed to be made out of the thicker plastic than the Zagg product.
I just put on a ClearCal from RadTech and it went on pretty good with no bubbles. I like that it is shipped flat not rolled up like the invisible shield and you also do not need to add water or whatever fluid the invisible shield needs. You get 2 clear screens or 2 anti glare screens for $19.95. I got the clear cause I don't like the dull screen look from the anti glare protectors. Here is the link.
I have the invisible shield on my iPad as well as my iPod touch. I was concerned about such a large surface to cover and getting air bubbles in it, which is a pet peeve of any screen protector I use. After watching a best buy employee put one on, I realized that it's not that hard. Just spray your fingers with the provided solution, spray a mist of it over your iPad screen, then spray the shield. Align the bottom edge with the home button hole, keeping an eye that it's straight along the edges and push the squeegee upwards, pushing the bubbles out as you go. It's really that simple. If you miss a spot, just pull the shield back a little and squeegee it again. I love it on there. I like the added protection and I REALLY like that it doesn't show fingerprints as bad as the naked glass does. The only thing I will stress is DON'T SOAK YOUR IPAD! Electronics and water don't mix. Let it sit for 24 hours to dry before turning your iPad back on. I didn't use the back shield because I have it in the Apple hard case. It's totally protected. It's a great product. (They use it to coat helicopter blades).