I bought this on the recommendation of a colleague who has the iPhone 4.
It is a hard backed cover, with rubber sides and the phone fits very nicely inside and feels well protected.
However, I had bought this for my new iPhone 4S and the mute switch is a little lower down than on the 4. Because of this, the cutout in the rubber siding for that switch just covered the bottom quarter of it. The switch was still accessible, but when I flicked it a few times, I could see that there was the potential for the switch to break over a prolonged period of time.
It was with regret that I had to send it back, as everything else about the cover was perfect!
So in conclusion - buy this if you have an iPhone 4, don't buy it if you have an iPhone 4S. I bought the Griffin Elan Form Graphite case, which fits nicely for the 4S - see my other review of that product -
Griffin Elan Form Graphite Case for iPhone 4 / 4S