I cannot speak for anyone else in the world, but in the hard, rocky and rooty ground in which I find I am frequently digging, you are a fool not to have a set of heavy duty posthole diggers with you.
The heavier the better.
You want a model with big blades.
I strongly advise people to not get a cheap lightweight posthole digger -- they cannot do the job, and you are better off buying a good shovel and not using a posthole digger at all.
The posthole digger pictured at right has two one-piece high-carbon steel blades that have pre-beveled edges for easier soil penetration. Extra large hinge bolts are used for improved durability. The handle is mounting with outside nuts for easy tightening. The round handle shanks are made of solid Ash, and the whole thing weighs about 17.5 pounds.
Some heavy-duty posthole diggers now come affixed to stiff fiberglass handles that are lighter than the wood ones. These work well and are easier to carry, but the handles have to be stiff fiberglass, not the flexible variety, or else they will spring too much and absorb the impact when the posthole digger is driven into the ground.
A posthole digger with fiberglass handles will weigh about 12 pounds as compared to about 17.5 pounds for a heavy-duty wooden-handled posthole digger of the kind you can get at Home Depot.
Hand tools tend to gravitate towards success, and the heavy-duty "Hercules" design shown here has stood the test of time. This type of posthole digger is the same style used by telephone pole installers when they are required to dig very deep holes by hand. The models sold by Peavy Tools come with handles as long as 12 feet, but a regular five-foot handle is fine for terrier work.