i can attest to shieldzone's amazing and abolute best quality around. you wont be disappointed.
i had it on my previous LG VX8600 and it was amazing. i just wish they had the FULL wrap around when i bought their shield for that phone.
one thing you might want to know is your screen might get all nasty with fingerprints now (especially with the iPhone), that WONT change, you just wont be fingerprinting the stock screen, rather the shield will get all fingerprinted and bleh. just wipe it off normally
as for scratching the case and whatnot, it wont. period.