It works, follow directions.

It does work, with the use of common sense, reading comprehension, and competency

Well, I was hesitant about buying the app after reading the reviews, although the concept is solid, I was unsure if the app functioned as described by the developer. To test the app, I stood directly in front of a mirror to measure the height of my eye, using a tape measure (and taking into consideration the 3" dimension of the tape measure) then entered that number into the app. I measured 14 from the wall and held the iPhone 4s at the proper height and distance... being cautious not to move the lens from that spot, but it is very sensitive to movement. The distance shown was two feet off from the actual measured distance, and one foot off from the height. Fiddling with the height that I initially entered (eye level), I changed it in the app to 14" less than my true height (head to toe), but kept the phone held at eye level... Now the app worked almost perfectly. The trick is to test/compare/adjust until the calculated height/distance match the real measurements, and the steadier your phone is, the more accurate it will be. Also, zoom in all the way while you are getting the measurements! Trust me. It only takes a few minutes to mess around and get the hang of it all.
I will test it further, if the developer updates the app, allowing lens height adjustment as high as 200, which would make it useful for longer distances, and at different elevations from the measured object. Please update it!

My trick may only work accurately at the distance I adjusted it, but I shall see... Ok, I tested it at 20 and the distance and height were the same, so it does work. It may fall out of sync if youre too close or too distant from the initial "sight in" adjustment.

Another update, or separate app, should include a man sized figure, moose or rabbit silhouette , at progressive distances. Ok, imagine a human silhouette on your camera screen that is say 1/2" tall and equates to a distance of 125. The 1/4" equates to a distance of 250. And 1/8" = 500.
Its not perfect, but it would work reasonably well to judge a distance. You would need to define if full zoom or no zoom should be used, as the change would affect the silhouette accuracy. Heck, even a vertical & horizontal ruler like L or + or a square, with line markings and a label for each one, and what height/width it equates to when held next to an object. Think range estimating mil dot, or bullet drop calculator.

PathfinderLFD about Point & Measure