Is there a quality difference between the unofficial iPhone replacement screens and the official?
I purchased one of the cheap replacements and it seemed identical during installation and right afterward, but it soon became apparent that the oleophobic (oil "fearing") coating  was either missing or very different. My screen is constantly filmy and gross and is extremely difficult to clean. It will come clean after a lot of buffing, but within an hour or two is back to being gruesome.

(I'm at the end of my contract and will soon replace it; otherwise I'd spring for the full-priced one and swap it out in an instant.)