[ghaff]

>Judging by some screenshots and appstore comments I assume its only roughly accurate, and requires you to move your phone slowly between walls in straight lines (not via your pocket or around furniture for example).

The site claims it's accurate to within 6 inches which, based on a quick trial, seems about right.

You don't have to move it in a straight line as far as I can tell. I just moved it around the room and pressed it against the walls.

Like you, I wouldn't have expected this to work. But it pretty much does. I didn't experiment with what radio signals it needs to work. (Though for most uses I'm not sure it's easier than a laser distance meter and some graph paper.)