I noticed all the comments regarding Dura-Blocks and thought I shoud reply. I am the President of Trade Associates, the parent company of Dura-Block.
Dura-Block makes both the hook and loop blocks and the PSA (pressure sensitive adhesive) blocks. There are 22 items in total including 3 kits.
Many people expect the Dura-Blocks to be perfectly flat and to sit flat on a table. The fact is that the blocks are not ridgid. They are "conformable" blocks which can be bent to conform to the shape being sanded. We do make one block which is of a different material, which is hard and perfectly flat and does not conform.
While most sanding blocks are molded and therefore do not have true surfaces, Dura-Blocks are machined to a flat surface. It is this flat surface combined with the material ability to conform that makes the Dura-Blocks special.
I hope this helps the discussion and that you will understand that I am just trying to provide info and not to promote.