I have several of Don's headrests as well as one specific for the carbon seat that Larry Graham sells.  You can find his info on the bacchetta forum.  John Schlitter uses one of Larry's headrests.  
 
I am currently using one of Larry's headrests on my Aero, and one of Don's headrests on my Xstream.  Both are carbon seats.
 
Differences...
 
Don's had a removable headrest (velcro).  Good and Bad.  Good in that is is removable to easily slip on a seat bag.  Bad in that it you push too hard to one side, it will pop off.
 
Larry's is a sigle piece, non removable headrest.  I use the bacchetta OFA bag on my aero, and it easily slips over the headrest.

I have about 90 miles on the headrest and i have learned a few things.
!. Now I can't live without it.  (on the bike anyway)
2. Pedal in circles or it will hit you on the back of the head.  (or at least sit up straight )
3. At first I would get hit on the back of the head every time I hit a bump.  I quickly learned to sit up when the road got rough and it became a non issue.