These are essential if you are building a drilling rig. I used this item with a holder. It's great that it can be used with other clips to place accessories.
The handles are not tight enough on the rails even when tightened. The screw at the top of the handles has a round hole, so you can't adjust the tightness. Why? Beats me. Wouldn't recommend it.
Considering the price I have to say they are incredibly well made. It's a little tricky to keep them very strong by hand tightening, but you can get them to hold up pretty well with a wrench. They also turned out to be surprisingly more comfortable than I expected based on the photos. Highly recommended!