I just bought one from the TSC, made by farm force, with a 9" auger with the spiral tip, this was my first time using one for the tractor and what a life saver. Just drilled about 20 holes this past weekend, no problems, except for a few larger rocks down too far to hand dig, and a few larger roots. No broken shear pins, and there is only one on this auger, bought a few extras just incase. Definately have to agree with drilling alittle than moving the tractor forward, the first few were angled, also let it drill alittle than pull it up alittle than lower and lift, and repeat until hole is done. After I hooked it up for the first time, just wanted to see how it worked and dropped the lever and it drilled right down, could not pull it up, not very fun, digging it out by hand.
Before you leave the store make sure the owners manuael and all hardware is there, I checked mine before leaving and had to have someone take all the hardware and manuel from a different one, I would have not been to happy when I got home if I had not checked.
Good Luck