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The zagg for iPhone 4 is not as good as the Body Guards for iPhone4 i did a review of the bodyguards and one coming up on the zagg bodyguards are way better
 
I haven't seen one in a while, but my friend had one on his 3GS and it wasn't that great. Feel was tacky and had the orange peel effect. I can't remember if it had the rainbow effect, never looked at it outdoors.

I would definitely recommend SGP, using Oleo on front and matte on the back. Great products.
 
I don't like its orange peel that makes the iphone look like a toy. I just took it off and put on a hard shield and it now looks much better. BUT this isn't your concern, so to just answer your questions:

1. i had no scratches to begin with

2. very easy to install for me, easier than the hard shield that looks better. The tiny bubbles or dust left under the Zagg shield at installation really heals themselves in a few hours.

3. very durable. I got mean the day before taking it off and used my fingernails to scratch the surface and pressed into the shield just for a test. It was not nearly broken. There were some nail prints afterward that healed in no time.

I put on a cheap case over "Zagged" iphone and the front skin peeled along the edges. It wasn't bad at all though. I just don't like the toyish looking it gives, that's it. Other than that, no problems.
 
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