There are differences between the quality of the shields. I can't speak for BestSkinEver, but I had an ApplePeelz skin for my Powerbook G4 and it was not good.
The problem with ApplePeelz was that the shield was just a thick vinyl sticker, so it was very difficult to apply. Even though I sprayed enough liquid to apply it to, there were many bubbles that never came out, and liquid still "trapped", weeks after I applied it. Plus afterward, the feeling of the clear vinyl was not pleasant, especially on the wrist rest area.
On the other hand, the InvisibleShield went on much smoother on my Macbook. The weren't as many bubbles in the process of laying down the sheets, and I think a large part of this is because the Invisible Shields are more "flexible", so they are more rubbery. The downside to this is that over time they will move a little bit in whatever direction they are stressed. For instance, my MacBook wrist rest area started moving 2mm to the left because of the pressure from my palms. I ended up removing the shields inside and just keeping them on the top and bottom.