Thanks for the Boot Camp article, arty is difficult to use but extremely effective. I'm still having some problems in using on-map mortars. First, their low ammo supply, 3 German 81 mm onmap mortars get 70 or so HE, while one offmap has 150 for less points.
The smoke produced is even worse. The onmap would seem to be more flexible, but their slow speed and reliance on a HQ spotter close by make them almost impossible to use offensively. They also have an annoying tendency to ignore their spotting HQ when some other HQ happens to flee or pass by near them. In an attack situation, onmap mortars seem to me to be waste of points.
Then again, seeing a 2-inch mortar knocking out a Puma in the Arnhem campaign was a sweet sight..