Even with a scratch you will never see it when the screen is on and you won't feel it. When the screen is off you will see it but it's not that big of a deal.
I just had a Photographer over to start shooting pictures and he asked me how I decided to get into the business. I pulled out my GNex and said that it was slippery as hell - took the case off and promptly (not on purpose) dropped it - on hard tile and it wasn't pretty. Now I have a small scratch on the screen protector on the right side but when the phone is on it's invisible and if you run your fingers over it you won't feel it. I'm very happy I had a screen protector on otherwise it may have broken the glass.
This kind of protector will get scratches easier than some others because it doesn't have the scratch proof coating. The coating that places like ZAGG use make it unclear and plastic feeling. However since Just Like Glass is incredibly hard it's very hard to scratch it to where you will feel or see it when using the phone. You could drop it on rocks and the glass likely wouldn't break. (I refuse to test it on my GNex). It's similar to the coating's they put on race cars to make sure the glass won't break if a rock hits it going 90 miles per hour. And since it's perfectly clear it never impacts vision.