Think of it this way. Even if the gorilla glass holds up great, there will still be scratches. When you get tired of the glass scratches and you HAVE a glass protector, peel it off and spend another $25-$40 on a new one and, voila! a screen that looks new. If you DON'T have a protector, then you are out $200 to replace the digitizer and the glass (and who knows what results you'll get). OR, you burn $100 on a new phone via insurance. OR WORSE, you just live with the scratches, which, if you're as uptight as I am about that stuff, can be nerve-wracking.
I bought the Zagg one. I had one for my S4 and LOVED it. Not only is it scratch resistant, but it has an oleophobic coating that resists (doesn't PREVENT, mind you) fingerprints, so the screen actually gets far less nasty. To be honest, I bought the Verizon-branded tempered glass protector at the Verizon store along with the phone for $24, and after two hours with it, I replaced it with the Zagg model, because the VZW model appeared to ATTRACT fingerprints. I'm MUCH happier with the Zagg.
As for the clarity and feel, I can say that the screen on the S4 and the S5 look exactly as they do unprotected, and the Zagg InvisibleShield Glass model actually has a smoother glide than the bare phone glass.
I'll never buy another phone that can't be protected with one of these!
Mark