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I have only the Frequency, so I can only speak to that case:

Pros: slim profile adds little bulk but makes the phone easy to grip. NGP material has decent resistance to stretching, and the case feels secure on the phone. Volume and lock buttons press easily and feel natural.

Cons: back edges of the case were hard and not comfortable to hold. I ended up using a fine grit file to sand down the edges, and it feels fine now. Still, I was pretty annoyed that I had to mod the case to get it to my liking.
 
PixelSkin HD. I did not like the feel of the Frequency when I felt it. It seems like Incipio's NGP/Frequency case is of much less quality than their 4/4S NGP case.

Plus, Speck makes phenomenal products.
 
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