For a fiver, this is a bargain!
I was really impressed with the mechanism - the arms on the side are adjustable, and have sponges to stop them damaging the phone at all. It would also hold many other phones/devices tightly, so long as they weren't much narrower than the desire. As well as the arms on the side, it also has a couple of flip down supports to hold it from the bottom.
It gives a really tight grip on the phone - once mine was in, I've been able to confidently hold it upside down and shake it all over the place and it doesn't budge.
The only reason I've given it 4 stars rather than 5 is that the windscreen suction mount fits into the phone-holding part with 4 plastic hooks slotting into the grooves on the back. I'm not sure if mine was a one off, but there just wasn't quite enough clearance for the hooks to fit in the grooves. But I just wore down the corners of the grooves with a screwdriver and now it goes in fine and grips very tight.
Despite this small issue, I'd definitely reccommend!