New screen protector. It was only a matter of time before the Zagg invisibleSHIELD protector wore off my iPhone 4s. Total time was about six months, and it didn't fall off so much as I got tired of it not sticking properly that I just peeled it off. I'm not disappointed that this stuff doesn't last longer, it's the prices that they charge that I have a bigger issue with. The invisibleSHIELD actually did quite well until I left the phone in a couple of hot steamy places. (If you must know, I was showering with the phone playing podcasts. Happy?) I think the heat degrades the adhesive backing of the product, making it much easier to come off. It never fully loses it's stickiness, but what ends up happening is that the corners will flap around freely if you rub them the wrong way. A firm press and it comes back into place, but by then, it's pretty much the end.
In the picture below, you can see by the discolouration that the edges of the protector are the weak spot. No amount careful preparation can keep lint and stuff from creeping in under the edges of the protector, and after a while, the adhesive gets gummed up and loses its stick. However, the product left no residue on the phone. The stuff does live up to it's billing, even used, it's really tough and pretty much impossible to tear by hand. Puncturing it with a sharp object is not as easy as it seams either.
A local telephone store I wandered into had the iShieldz brand instead of Zagg. (Geez, with a name like that it's like they're intentionally trying to repel adults). This goes for cheaper and the product range covers less devices. If you go online and try to find reviews for the various screen protectors, you'll be sure to find a a tonne of negative reviews pissing on the various products out there. It's so bad that I'm of the opinion that online reviews for these things are pretty much useless, the various companies have gone out of their way to have shills clog up the forums and comment sections of Amazon and various other outlets.
That said, I'm of the opinion that iShieldz is a better product that invisibleSHIELD. First of all, it goes for cheaper and is less marketing driven. For a pack that covers the front and back (but not sides), the retail was $25CDN. I just outright asked the clerk if they would bring the price down and he knocked off $5 right away. That's what you can do if you don't go for the big name product. iShieldz is also somewhat easier to apply. The supplied bottle of application fluid is something alcohol based in, which makes the drying process quicker, and also helps dissolve any remaining fingerprint oil you might have got onto it. I'm almost sure that the stuff Zagg gives you is nothing more than de-ionized water. However, the iShieldz cutouts are not well done... I had to really pick at a few corners with a pen knife to get it to lift cleanly from the backing paper.
The product itself has a smother feel than invisibleSHIELD. It's not as thick and doesn't have quite the same gel squishiness. When properly applied, it also doesn't have a pronounced an 'orange peel' texture, but neither product is particularly bad in real life use. You only notice it if you spend a lot of time looking at your phone edge-on. iShieldz does pretty much the same thing, it's self-healing, so nicks an dents smooth away in a matter of moments. I'd say that it doesn't really matter which brand you get, real world use renders them disposable in the end, so pay the least that you have to.