I use these and like them. I noticed the same thing that Petzl warns of. If the ascender is rotated, the cam does not grab immediately. In my experience, it has always grabbed, but not until loaded. The little time lag definitely increases the sphincter tone. I still have my lower ascender on the rope, so I have little fear of falling.
As a side note, I place a biner in the upper hole around the rope. I did this even before I read it in the instructions, not for the safety factor, but as a required extra step before switching from ascent to descent.
I use the Petzl Stop and really like it for descent. I rig the descender, load it, lower my upper ascender until I am hanging on the descender, then remove the safety biner. I do an extra check before removing the upper ascender.
That's my story....
Bob