We didn't have one of these. Instead, I got to crouch on the ground and hold the post with my hands while my father brought the front-end loader bucket down on it. Good times.
When I first started in electrical work, we'd take turns with a sledge hammer. Then we bought one of these, and it cut the time in half. Now, I use a driver bit for my demolition hammer and it's done in about 30 seconds. The driver bit also works well for fence posts, just sleeve a 24" section of EMT over it.
Man, I hate driving rods with anything other than a hammer driver. I live in the Appalachians and every damn time I hit some gigantic river rock or some shit, its never just 8' of clay. That post-driver is a step up from a sledge like you said for sure, but imma just hop on an excavator and shove the bitch in if I can these days. Too old for the dumb shit ;)