I worked with a guy for a few months that had one. Though I'll admit I was quite....impressed- by what that thing put up with, it was scary to work around. He would bomb LARGE pieces into all the time. Just scary, scary stuff to see.
That said, HECK YEAH it was useful. There'd be guys over by the truck cutting large pieces of wood and loading them into the truck (sold later as firewood) and boss man would come along and we'd lift entire pieces onto the trailer and cut them up there. Saved a TON of work.
Another neat use I saw, I'd be up in the tree, on a lead overhanging a house, rig a piece to be lowered, cut it free, the ground guy would hold it, then the bucket operator would toss me the hook, I'd clip the hook to the lowering line, then he'd pull the log away while the ground guy let slack out of the PWIII. When he got to the drop zone, the ground guy would lower it down. It was pretty smooth, actually, and let the tree take the loads, rather than the boom!
Check out the picture....I think I get the award for BEST PAINT diagram of all time!
love
nick