We use them on our off-road tandem; they are quite nice.
http://home.att.net/~mark.livingood/...2/ecdm221b.JPG
Can't offer too much in the way of independent testimony relative to shock reduction given that our front fork provides 5" of spring/oil dampened travel. But, that said, they are incredibly light, incredibly strong, and have a virtually unlimited fatigue life. Well, they look pretty cool too. Speaking of strong, Easton has a very cool web-based demo on carbon bar strength...
http://www.eastonbike.com/COMPONENTS/bar_test.html
I have thought about fitting some of the carbon road bars to one of our road tandems for the "cool" factor and some gram shaving but they are still too cost prohibitive to be an attractive choice and some of the features like recesses for under-tape cable routing haven't made it into Easton's molding processes.
http://www.eastonbike.com/COMPONENTS/bar.road.ec90.html
Of course, some of the more expensive models (FSA, Deda, Stella Azzurra) do have these features:
http://www.bicycleexoticadirect.com....Handlebars.htm
Moreover, if I was in the market for something to give me a more compliant ride / less fatigue and didn't yet have a carbon fork on my tandem that's where I'd invest my scarce upgrade dollars. Given what a pair of carbon drop bars cost (well you certainly would want your stoker to have them too!), I believe you get a lot more bang for the buck with a carbon fork upgrade, e.g., significant weight savings, stiffness, and shock absorbing/fatigue reduction qualities.