I've tried them on several bikes, but never needed one until I got down to the 18* recline that I had on my tica.  My teammate Chris had one on his carbent but I've never found them to be all that cmofortable.  That may be because I have a really long torso though and the headrests have never fit.
 
plenty of folks use them though, so you can find one that works.  I can ask Chris which brand he uses for the carbent, though if you want.

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Yes but no answer yet.

Have a few bucks and a little ingenuity? I made mine out of 1/2" CPVC tubing and a forearm rest from an old aerobar. I've used it for the last 9 months and about 8k miles. I made it for the florida trip and it performed far better than I ever dreamed it would.  I can almost always use it continuously, including the entire 26 hours of the hc600. Only if the road is very rough or approaching intersections where I need to sit up do I have a reason the lift my head from it. The CPVP tubing is far more flexible (yet sturdy) than a metal support and the forearm rest fits in the back of my skull right where my helmet strap fits so it holds the helmet in place even in very strong headwinds, etc. Since it is flexible, it bends out of the way a bit when I'm cranking hard or when I hit a big bump expectedly. I used a neck rest with the EM seat before I bought the carbon seat but I was sitting more upright then and didn't need as much support. Before the big trip I knew I'd need something that would work all day long. The carbon seat is at the second is set for the 2nd hole from the top so it is almost completely laid back.
 
Basically the CPVC frame is a long rectangle shape with two CPVC tees separated by about an inch on the bottom with two long tubes that run up to an ell on one side of the top and a tee on the other. The tee on one side serves as my 'receiver hitch' for accessories such as an extra taillight or camera mount, etc. The forearm rest is the type that bolted onto the bars and the aerobar screwed into the block that attached to the bars and the forearm rest bolts onto the top of that. Just pull the clamp block and rest off and bolt it the short CPVC pipe that connects the ell and the tee at the top. I used a nylon strap through the two tees at the bottom that wraps around the seat plus a clamp at the top of the seat to hold it in place.  The unit fits on the front of the seat. When I have my headbag attached, I really don't need to top clamp. I only added to or the hc600 race since I wouldn't be using the bag.
 

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Paul, I knew you would come through for me.

I found his web site. It says the ADEM is for the mesh seat so I sent in a message for clarification.
Thanks

Okay
lets talk.  My cell is 214-783-3181
John

I have a ADEM-2C coming soon.  It is one of the first shipped by Don. I will post how it worked out.