You'd have to be mad to ask directions from anyone in Ankh-Morpork, so obviously a visitor to the city needs a Mapp showing all the places to avoid at all costs.
I bought mine because I'm contemplating running a Discworld RPG game and everyone knows y'can't play without Ye Mapps. Thus I was spared the angst shown by some down at the bottom of the review list who feel It Shoulde Notte Have Beene Donne.
I love the fact that the Mapps look like UK tourist maps I've owned, with a sturdy, book-like cover, a gazetteer glued to the inside of the left cover and the map attached awkwardly to the right one so that the weight of the gazetteer fights you if you try and open Ye Mapp outside or one handed. Ten out of ten for verisimilitude.
The mapp itself is (of course) a work of art well worth the cost of ownership, printed in glorious colour on glossy paper. It's huge. Should you be adrift on a raft it should make a very serviceable spinnaker.
The accuracy is a matter of heated debate, but it's not a debate I involve myself with because a) It is a mapp of an imaginary place which stands on it's own merit, 2) Pratchett was consulted by Briggs often during it's production and therefore obviously approved of its production and $) If you think the idea is doctrinally flawed you shouldn't buy it in the first place. If you've already bought it your choices would seem to be to not look at it, to sell it or to gouge out your eyes.
Personally, I enjoy looking at it, and I'm thinking of getting another copy to mount on a backboard so I can hange yt on ye wall.
It would be an excellent prop for people contemplating running the Discworld (HC), as would The Discworld Mapp.