I've had this cover for both the iPhone 3G, and now for the iPhone 4. For reference, I've also owned other variations of similar products including BodyGuardz brand, and also very cheap knock offs. I prefer Zagg for several reasons.
First, the installation is very simple. It seems other reviewers have had problems, but if you follow the directions it is fool proof. There is no adhesive on the product, you wet the cover with the included solution, and then apply it to your phone. No worries if you don't get it right the first time, you can simply lift it off the phone and try again. The key is to make sure you get it wet enough. After that, you push out all the air bubbles with the included tool, and let your phone set overnight. If there are still some little air bubbles left, they go away after about 24 hours.
Second, the protection that this skin offers is fantastic. I don't like bulky cases that take away from the sleekness of the iPhone. I keep my phone in my pocket with my keys, wallet, and anything else without worrying about it scratching. I don't know what the material is made of, but it's super strong and won't scratch. There's a lifetime warranty on this product too, so if you're actually able to make a scratch in this thing (which I doubt), they'll replace it.
Overall, I am extremely happy with this product, and will definately buy it again for my next phone. It doesn't ever peel or come off of the phone, it's never scratched despite the abuse I put it through, and was very easy to install. Great value, considering it's protecting a very expensive phone.