I have it for my 15" PB, and I use it all the time.
It's really thin, and it has a soft sort of feel, like silicone perhaps. The only thing I don't like about it is you need to wash it with soap and water about once a week, and from an angle it's hard to see the keys. I wouldn't recommend it if you can't type well.
iSkin says you can shut your PB's lid without taking the iSkin off ... technically yes, you can, but do not do this!! I did this, thinking I was doing something good by protecting the keyboard from touching my screen ... instead, I had tons of keymarks all over the screen (I didn't take it anywhere, it just sat on the desk like this). Luckily, I cleaned it immediately and everything came out. Since iSkin isn't flat, I would not recommend closing your lid with the iSkin on. Others might have a different opinion, but that's my experience.