Appears to be top quality acetal compared to other buckles found on my gear. At room temperature they are a bit harder to release than most side-release buckles. In below freezing temperatures 10-20F I found my fingers hurt when opening them, and I'm a climber. I had two of these for one side of my HMG backpack's compression straps, and two others on the other side. When the nylon webbing became wet and icy I was able to tighten the side with the original buckles with one-hand, but the webbing wouldn't slide through these buckles. Two hands were needed to adjust.
On the plus side, they do seem to be more durable. I've already broken three of the original ones that came with that backpack.