Used it, and must admit it became a pain...
Theory is excellent.... Trying to find a scanner that will bring in a decent size map is near impossible. Then you have to go and mark up known points on a map and the more you do, the more acurate it is. One of the tests I did was to take a photo of an OS map (one whole side) but taken from as close to above as I could get. this meant the map tapered in at the top (As it got further away from the camera) and it did work after I entered the points for roughly 9 points on the map, not 100% acurate, but near enough.
I ended up on Memory map (OS edition). Maps already there, Acurate and far easier to use on the PDA IMHO.
What you trying to do with it? (In car Nav, or just plotting courses to send waypoints / tracks to your GPS ?
EE