There is no question that a post-driver will be less work than a post-hole-digger followed by backfilling. If you have access to a post-driver, by all means use it. All you do is drive in the post and then... well, that's it. Walk away. If you have three guys, one can drive the tractor, one can drive a truck with a bunch of posts in the bed, and one can walk along setting the posts. Bam bam bam. Get a rod that you cut to length so you know when the posts are driven in to the appropriate depth, or make a mark on the driver, or something like that.
The only question is how big of a driver you might need, based on how tough your ground is. For that, you would have to ask somebody else, or go direct to a manufacturer. But there is no question that someone makes a driver that can do pretty much whatever you need. I have seen a demo where the biggest driver a company made put an unsharpened railroad tie into the ground.
One small disadvantage of the driver is that if you put the post in crooked, you're kind of screwed. With auger-and-backfill, you have some ability to tweak the post during the backfilling to make sure it's level. But all things considered, if I had a lot of posts to do, I would prefer the driver for its speed and efficiency.
PS: In the interest of full disclosure, I have never personally used a driver, but I have used an auger, and I have seen demos of what those drivers can do, and it is simply awesome.
