I just put a screen protector on my Galaxy Nexus, have had it about a month. No scratches yet thankfully, I wont keep it on there long, just wanted one for a while.
I have never put a scratch on any of my screens, at least since I got my g1. Well until I got this phone, I was using my wife's iphone 4s before I got my Nexus. I put one little scratch on it, only could be seen under lighting. Her iphone 4 was never scratched, and the iphone 4s never was until then either.
I am very careful with my phones, so I am not sure how it happened. Sometimes I think it is just a fluke and it happens.
The quality of this screen seems as good as the others I have used. I wouldn't worry about it to much, but sometimes it can just happen.