I have drove ground rods into some really hard clay. The kind where some bruno looking fella comes over with a huge sledge and nails it dead on and the rod never moved...
The DeWalt rotary hammer (the big one) with a FORGED steel rod driver (The cast iron one kind of sucks and I have broke em, the forged is awesom) seams to drive it pretty well, sometimes goes kind of slow, but unless you get a deep drilling rig out there:-)
