I went through two InvisibleShields for my iPhone 4, each lasted for about two weeks and eventually "rolled away" after putting the phone in and out of my pocket a lot (these were cargo shorts too).
Not sure why I bought another one though, I really wanted to like ZAGG, but their "orange peel" texture makes way too much friction for me plus the screen keeps coming off.. especially if you have a pretty tight case on it.
Just my two cents. I've went with generic-ish (like $10) one-shot (aka dry application) screen protectors and that one has lasted for over a year now (so it also depends on how the phone was treated).
Zagg = Beats. Don't buy into the hype!
Some people like them though, but we're all entitled to our opinions.