I've had a couple of rubber/silicone cases for my iPhones (from Incase and Paul Frank, as well as a Tunewear one briefly) and they stretch out over time and I didn't think they'd hold up at all with repeated removals/insertions.
I have the Incase Slider and it's perfect if you dock your iPhone frequently, which I do. It has not scratched my iPhone.
i had the Griffen elan and it looks and feels great. but if you are not careful when you slide it in, you can eventually start seeing fine line scratches on top (where the clip holds on to your phone) and the back bottom area (where the sleeve holds your phone). right now i am using the contour showcase and i keep the incase power slider in the car. the power slider can scratch your phone a little on the back and the showcase is great but you have to remove it to dock.
I have an invisible shield and a marware sportgrip case. I take it off a lot and it doesn't stret h and I don't have to worry about scratches.When you drop most twopiece cases they tend to split open and almost fling your iPhone out. I suggest one piece cases.