I am an IS believer... had it on my TREO 600 and 650, and since I switched over to the iPhone.. it also has IS, BUT this was after I used 2 different cases already, first was the iSkin revo for iPhone (red rubberized cover) and then the Contour Showcase.
Out of these 2 hard cases, I liked the Contour Showcase better because it shows off the iPhone design a lot more than the mostly covered up iSkin Revo. Both cases have great shock absorption. But now I have the IS film because I am sick of the bulk from these hard cases. I want my iPhone to be slim and shiny again, so I bought the IS..Protection against scratches is great, and the only downside is I have to be really careful since drops will do damage (breakage).
Installation is not hard, but you have to be patient if you want a no bubbles- seamless look. Mine has virtually no bubbles. Comes with front and back and 2 pieces for the front chrome, just use the spray that it comes with and you should be good, work 1 section at a time, squeegee out the solution and let it dry then continue with the rest. Corners are a bit tricky since the flaps are harder to lay on the round corners. Be patient and you will love the IS... and your iPhone even more!