Well, ZAGG offer the Lifetime warranty themselves. I've replaced many of their products after getting dirt underneath or changing to yellowish colour (6-8 months of usage) and they replaced them FREE of charge.
I've been using InvisibleSheild on my gadgets for couple of years and I'm so happy with their protection, how they look on the phone and their service.
But to be honest, I didn't find the SXS InvisibleShield (Maximum protection) to be as good as they were for Arc or X10.
They don't seem to cover the corners (where normally the phone gets most of the hit when falling) And they seem to peel of much faster than previous versions of ZAGG products. Started peeling in less than 2 weeks and eventually have collected dust underneath (especially the corners). I also noticed that it doesn't fit exactly to cover all parts of the phone as I can see many areas of the phone unprotected. And believe me I do have experience in applying these products as I've been doing it to all my gadgets (Xperia X10 - 3 times, Xperia Arc - 3 times, MacBook Pro - 1 time, PS Vita - 1 time and Xperia S - 1 time).
So, as an expert with ZAGG products I'd say this is the worst product I've received so far (Screen protecion is great though). Not sure if I was unlucky to have received one of the "faulty" products but this is my experience so far.
And if you want to use ZAGG product for SXS, then I'd suggest getting the screen protection only of the full protection (screen and back) rather than the maximum protection which doesn't seem to be as good as it used to be!
Forgot to mention that I applied the InvisibleShield over the original screen protection of the phone and didn't notice any issues with responsiveness (maybe only a bit harder to swipe fast when your hands are wet/sweating because of the nature of the InvisibleSHeild material).