I love my iskin. It doesn't affect key movement at all. It's very thin and doesn't smudge the screen. The three main reasons I bought it: I get a lot of hair in my keyboard (my hair seems to fall out rapidly, despite that, I'm really not going bald), very hard to clean; it keeps finger oils off the keys and so cuts down on grime(the rubber somewhat absorbs oils so it feels great typing even when it hasn't been washed in a while); and I changed my keys to a dvorak layout, so it gave me home key ridges back. 🙂
I would definitely recommend it. Apparently, some don't like the rubbery feel, but I actually like it. It makes the keyboard feel a little more substantial. Finger oil will ruin the original finish of your keys, the iskin will stop that. It's actually "wicked" the oils out of my keys that I couldn't get off with a mild soap bath (I popped my keys off to clean them and to set them up as Dvorak). My keyboard -almost- looks new.