Wife and I drove 63 fence posts this weekend. Probably a total of around 8 hours of work. It would have been a little faster yet, but the ground was still a little wet and muddy, which is not ideal conditions to be operating a skid steer. Now all we have left is bracing the corners, hanging 600' of woven wire fence and installing 5 gates. The sheep will LOVE all the extra room they will have.
That post driver sure makes things easier. Quite a few years back I did a small area for the dog run with a post hole digger. Not fun! I got smart and picked up an auger for the tractor a couple years ago at an auction. A much more pleasant experience.
The rental on the unit was a little pricy up-front, BUT I can't imagine doing it any other way. Thankfully, and quite surprisingly (look at the background in the picture in the OP), we only ran into 2-3 rocks we couldn't pound through.