Check out the amazing in-depth reviews on
iLounge for the best information.
But, basically:
InvisibleShield/other protective film:
Pros: Good scratch protection, no added size or weight.
Cons: No impact protection
Skins/rubber cases:
Pros: Good scratch protection, good impact protection.
Cons: Adds size and weight, attracts dust.
Soft cases:
Pros: Good scratch protection, ok impact protection
Cons: Adds size and weight.
Plastic Hard Cases:
Pros: Good scratch protection, ok impact protection,
Cons: Adds size and weight, easily scratched.
Metal Hard Cases:
Pros: Good scratch protection, Good impact protection.
Cons: Adds a lot of size and weight.