I purchased one and it arrived today. I bought it for a Pentax film camera, but I tried it on the K10D just to see if it would work and it is OK on the K10D. I tried the Sigma 170-500mm on the camera with the QR plate on the camera and after a couple of tightenings, it held without twisting when the head was in the slot and the lens out horizontally. The unit came in two very nice boxes and without instructions or tools. Although it doesn't look like it, even when looking at it directly, the center hole in the mount accepts a small Allen wrench. I don't know the size, but it was one on my keyring set. This is important for getting the unit tight. You can use a pry bar through the loop, but the screw hole will probably be a little off this way. The reason why the fit of the D ring in the QR plate is important is because I needed to add a tiny gaffers tape flag to the D ring to be able to get to the D ring when it was flush to the mount. If there is a way to pull it out without this flag, I haven't found it. The unit is closely machined so there isn't much room between the D ring and the QR plate so I needed the QR centered around the D ring for my flag to work well.
The slots in the sides of the QR plate work really well with a hand grip/strap. There is plenty of room under the slot for the webbing. The D ring and the hook are both well made and seem to be pretty heavy duty. I don't have any issues with its intended use. There is a cork pad on the QR plate and once tightened I don't think it is possible for the unit to work lose. Holding the 170-500mm sideways on the mount convinced me of this.
If I had an R-strap, I would definitely use this set up for the QR plate. As I don't have an R-strap, I used some
climbing 9/16" webbing I bought at REI,
$1 bag of 3/4 buckles, and a shoulder cushion I just off another strap to make a Russell-strap.
Thank you
Russell