Obviously, when perspective comes into play you're going to get different results, and help files in CamRuler are careful to point out that it can only accurately measure objects that are in the same plane with your reference object. But the problem is not whether the app works, it's how hard it is to use.
The app's touch-screen interface gets infuriating quickly as you try to size both the reference object and the object you want to measure. Resizing the circle is easy enough, but trying to get the circle onto the quarter in the image to check seems needlessly difficult. It was almost as though the app were resisting my attempt to get the circle around the quarter. I asked another CNET editor to try it, and she said it felt like trying to push two magnets together. I ran into the same interface limitations when sizing a rectangle over an 8.5x11 piece of paper. I could pinch to resize, or use a two-finger gesture to rotate, but when I took my fingers off the screen while rotating, another touch would reset the rectangle to its original orientation. This meant I had to rotate in one try.
The same was true of resizing a line to fit the length of the object I wanted to measure. On the iPhone screen, it took several tries, making tons of minute adjustments before I was satisfied that I had both ends perfectly measuring the length of the pen. But when I decided that I wanted to resize my reference 8.5x11 piece of paper by touching the area button at the bottom, the app gave me a circle instead of a rectangle. The only way out of this conundrum was to start over (there is no back button). Frankly, the more I tested this app, the more I thought it would be easier to drive to the hardware store and buy a ruler or tape measure to save myself further headaches.
CamRuler does what it advertises: it measures objects using reference objects that inhabit the same plane in the same picture. But with the app's poor interface controls you're going to make a lot of adjustments to measure just one object and at $1.99 (at the time of this review), using this app to measure something is simply not worth the trouble.