I was a bit wary of buying a set of these screen protectors after reading some of these reviews. Nonetheless, I needed to replace the one that came with my iPhone case after I fouled up the application and made it pretty much unusable, so I went with it.
I can only assume that the negative reviews came from people who didn't use the screen protectors in conjunction with a case, in which case it would have trouble staying on. I've had no such problems as the case covers the edges of the protector and holds it in place. Additionally, a set of five for less than a pound seems like very good value compared to what I was expecting.
With the protector in place my iPhone feels noticeably more comfortable to use, with less friction and none of my grubby fingerprints left all over the screen!
In summary, if you already have a case for your iPhone then a set of these protectors is a great investment but otherwise you'll just be wasting your money as they won't stay on in your pocket.