I have Automax, Centec, Whites Bullseye II, and of late, a Garrett Propointer. With no detecting to be done during this long Minnesota winter, and a bad case of detecting fever, I thought I'd do a little comparison between the four propointers. In an airtest with a quarter, the Propointer could detect twice as far away than the other three. Then I tried each pinpointer against my plaster walls to see how they did at detecting studs. Plaster is at least an inch thick so I thought it would be a better test - and it sure was. None of the the pinpointers, except the Garret could detect a stud. The Garrett would start to sound off a few inches away from the stud and let you know when you were right on top of it. The other three would even sound when they were right over the nail. (I marked its location after I found it with the Garrett.
That backs up my opinion that the first three are next to useless unless the target it sitting right on top of the dirt. Save your money and buy a Garrett. Plus the Garrett is the best size of the bunch. The automax is huge and I never could find something to carry it in without it dropping out every time I bent over.