I have a Rohrer 99E-W, their largest driver; as well as the compressor that they also sell. I bought it this winter, and am currently putting in 80 9ft x 3.5" steel end posts (3ft deep, at a 70 degee angle) on 40 rows of trellis in our vineyard. After I finish the last 20 or so end posts, I'll be driving 1200 8ft x 2.5" steel line posts (2ft deep, perpendicular). [When I bought the driver, I sent Rohrer short pieces of the posts that I was driving to make the adapter that holds the post. They were able to make a single adapter that holds both posts.]
Our ground is heavy clay with plenty of basalt rocks and boulders. In the summer it is hard as concrete, but in the winter with the 40-50" of rain we get, it softens up enough to drive posts well. After laying out the trellis rows, I ripped the ground about 30" deep to get the boulders out that were close to the surface.
I have a 3pt hitch pair of forks, which I put a pallet on to hold the compressor, hose, driver, etc. as I drive the tractor down the line. So far, with the end posts, I haven't had any problems. The driver drives the first 24" pretty quickly, then takes a little longer to go through the last 12". the only complaint I have, and it's my own fault, is that I am doing the work alone and the combined weight of lifting the 9ft posts with Rohrer's heaviest driver is a workout by the end of the day.
If you're only planning to drive t-posts, you'll be able to use their lightest model and the posts will be lighter. You should be able to put quite a few into the ground each day and not be quite as exhausted as I felt on the portion of the day I drove 60 of the endposts (which took some extra time getting them in at the proper angle), especially if you have a helper or two. One great thing about the Rohrer is that once you get a post started, you don't even have to touch the driver until you want to stop it.
If you choose to buy one of their drivers, make sure you have or buy an air hose oiler (Rohrer has them). You'll need it to keep the driver lubricated
I think the Rohrer video on youtube [ame=http://www.youtube.com/watch?v=rEAgOKem48g]Rohrer Manufacturing and Fence Post Driver Information - YouTube[/ame] accurately portrays the driver in action.
They named it right!
