Check this out: http://ventduvoyage.free.fr/Materiel.htm#voile
The page is in French only though. In short, these guys have started a world tour on Optima Condors equipped with sails! They started their tour on April 16. From what they say, the sail is very controllable, even in gusty winds, and the sail adds up to 3km/h to the average speed with 1h of wind-assisted riding in a 5h ride. The bike can go up to 12 to 15km/h on wind power alone in a force 2 wind, and is a great assistance for hill climbing. The total weight of the sail and mast is 4kg, it's made of alu but they're not sure it'll last too long, they reckon a steel mast would be better. They expect to use it 2 or 3 times a month, and they'll see if the assistance is worth carrying the extra weight the rest of the time.
Look at these great photos:
Sooo, I have a Condor, I have kite cloth, guess what I'm gonna try next?