I bought an ordinary silicone keyboard protector with my MBP and used it for a month or two. There was no way to get it clean, ever. It wasn't thin or transparent enough for me to read the keycaps unless I was directly above it. It quickly became permanently buckled and distorted by the repeated pressure of my fingers, so that it wouldn't lie uniformly flat on the keycaps. Thick as it was, it left oily prints on my display every time I closed the display.
Then I saw an ad for the Moshi protector, and immediately ordered one ($23 from Amazon). I took it out of the package, put it on and...it disappeared! I can read the keycaps easily through it from any angle. After four months, there is no physical distortion of the film; it still fits my keys perfectly and lies flat on them.
I took my MBP into the store where I'd bought it with the original protector, set it on the counter in front of the manager, and opened it. "I want to show you something," I said, and lifted the Moshi protector off the keyboard. His jaw dropped. "I couldn't see that at all," he exclaimed. "You should carry these instead of the junk KB protectors on that rack over there," I told him, and he agreed.