A bit stiff at first, but then takes on a comfy shape with wear. Modest 2mm taper, I believe. My 20mm happens to be off watch right now.
I'll note that the underside takes on small ridges. Doesn't bother me...actually helps air things out, as with a diver!
I believe @StephenRL has had the same strap on his Speedmaster lately, which looks great! If you go back to Tuesday's page 9086 of this thread, you'll see his.