Age and enthusiasm play a big part in how many holes a person is willing to do with a PHD. There was a time when I would have dug 15 fence posts by hand. Now one mailbox post is pushing the limits. I went out and got my PHD out of the shed the other day and one side of the scoop had rusted half off. Seems it was up against a bag of lime and ten years of sitting there did it in.
My back aches from just reading this discussion! I attempted to go shovel-only years ago, with disastrous results. I go power auger all the way now. Not only is it faster, the results are much cleaner as well.
I like using a spade for this. Once you get down so far, you can sit down and chip away a couple inches of dirt at a time and use your hands, and a post-hole digger to remove the dirt. It seems like a hard job, but it's really easy digging when you can sit down!! I dug numerous holes this way down to 40" inches deep--even underneath an existing deck that was poorly constructed in order to get more supporting posts under it. A spade is my favorite digging shovel.
Digging post holes is on my list of not favorite things to do, but over the years I've dug a fair number of them. The proper tool depends largely on the type of soil you're digging in. Our ground is full of rocks, some fairly large. I've always used a post hole digger and a digging bar for when I encounter a rock. Ever notice how the big rocks are usually pretty far down in the hole? In those cases the hole gets a good bit oversized but I don't know how you'd avoid that happening.
As I'm getting on in years I thought it was time to rent a gas powered auger the last time I had some holes to dig. It didn't work out at all because it kept hitting rocks which would throw it off course and move the hole out of line. A Bobcat with auger attachment does work, but that's way overkill for a few post holes.
The best way of all I found to dig post holes is to lay out where I want them, then hire a much younger man to do them. I have the equipment, they supply the muscle. This works so well it's my go-to method from now on.