I've tried this app and found the data to be pretty unreliable. Maybe this will change as they get more submissions, but for now it's not very useful.
The interface is not very well designed, either. Instead of using the pinch and squeeze gestures to zoom, you have to use a slider, which is a major hassle. And you can't scroll around the map to see what's ahead. You can only center it on your position.
There's another similar app called "iRadarUSA", but that one costs $15.