I got the red arm case. It is made of a flexible silicone that the 3rd generation ipod nano slips into. The silicon is a nice solid feel that would seem to give it some cushion if it were to be dropped. The case itself is open where the wheel, screen and port is, but covers everything else. The advantage to the openings is that the plugs can be accessed easily, the wheel can be used and the screen is not blocked by anything making visibility easier. I also use my Nike + with ease. The disadvantage to the case is that I run outside, which means that all the openings that make the case more convenient also means parts of the ipod are exposed to the elements.
The back of the case has a strap that feeds through it. The strap can go around an arm or through a belt...a nice function. The strap has sturdy velcro and a bit of flex to it, which I think makes it a little more comfortable around the arm.
The red case is quite similar on color tone to the red ipod. Red is my favorite color, that is why I picked a red ipod. previous cases have covered the color of the ipod, but this red is nearly the same color so I don't lose the "specialness" of the red.