I have used a Post Hole Digger (PHD) mounted to the 3-point hitch on my TC33D a number of times with 6" and 9" augers. In nice soil, no rocks bigger than a softball, and not a lot of clay or shale it works like a dream.
The fun begins when you start to introduce the clay, rocks and shale. By fun I mean breaking shear pins and having to use a large pipe wrench to twist the auger out. I can see the advantage of the hydraulic driven PHD because you can reverse it as long as you haven't broken a shear pin.
Are you looking to mount it on your bucket or the 3-point hitch?
To me the bucket of my TC33D or should I say the structure (hydraulic cylinders, pivot points, long arms) just doesn't seem sturdy enough. If you mount it on the bucket I would think when you dig (especially if you hit hard stuff) the entire structure is going to be dancing around which can't be good for it. Maybe on a bigger tractor this is not an issue.