I have a Shaver HD-8, w/hydraulic tilt and love it. I drove over 300 posts with it, and sure saved a lot of work. Line posts were 5", and corners/gates 6". Took a bit longer to drive the corners, as I put them down 4'.
I could drive 20-24 posts a day by myself in the short daylight season. That's jockeying two tractors, then moving and aligning posts, etc. With two people that can work together, you could probably do 2-3 times as many.
I actually bought mine off ebay. It was used to build several small horse paddocks, then, never used again. Just taking up space, so they decided to sell it.
I'd advise if you get one, which ever brand, do get the hydraulic tilt. My former neighbor at the other place bought a manual tilt one, and was forever cranking it to adjust it.
A small torpedo level is the berries to get, and keep things plumb when setting up to drive, and check the post when driving. I plumbed the driver H beam first, then plumb the post to match.
You'll not drive post here when it's really dry. I put all of mine in starting in late fall, through winter. Great thing is..., no tamping..!!