I've had my Leinbach Model 7300 PHD with a 9 inch auger for several months and am  very happy with it.  I run my little JD Model 670 at 1500 RPM and am able to dig down through our caliche quite nicely.  I have broken several shear pins because there are many rocks of various sizes--some big-- in the soil and I have to stop and break them up with a chisel bar and remove the pieces with a clamshell digger and a posthole spade.  I do want to tell you about one incident.  I was digging down through clay and it was a big damp.  I try to be careful about raising the auger and clearing the hole often, but I got a bit careless as I was finishing the hole at about 40 inches down.  The auger would not raise; in fact, it wanted to keep going down and when I stopped it, it wouldn't turn again.  After trying a long bar, a farmer's jack, etc. I finally had to bite the bullet and dig another hole alongside the auger in order to free it.  Boy, that's an afternoon I don't want to repeat soon.  I can tell you that I sure am careful now when I get down that deep.