I have the Moshi ClearGuard Keyboard Protector (as well as the Moshi PalmGuard) for my Macbook Pro and I am very, very pleased with them both.
In the beginning I found the Moshi Keyboard protector took some getting used to with typing but after a few days it's fine. It feels funny to type without it now!
Prevents dust, dirt, grease etc from ruining the keys. If I take it off, my keyboard is in excellent condition.
I would definitely recommend it.