Thanks for all the feedback, folks!
These will be replacing cedar posts that were put in 20-ish years ago and are rotting. My ground is 6 to 10 inches of good loam over clay, blue marl and pin gravel, so the holes generally make pretty good wells.
Since that original fence was built, I've started using a bigger auger on the digger, and back filling with screened gravel instead of the glop that came out of the hole. The theory is that it will drain better and cut down on the frost heaving the posts.
I have to REALLY work to get down past 3 or 3-1/2 feet on most holes, that's when I hit the pin gravel and you might was well try augering through concrete.
I'm on the wrong side of 60. If they last 20 years, I'm not going to be the one replacing them..."8^)