In general, a sail will be helpful up to windspeed but no faster, but really helped compensate for the weight of the bike, which was fully loaded with pack, camping gear etc. On uphills and in high winds it felt like magic.
Probably the most noticeable effect was in cities, where people of all ages and demographics would stop to stare, chat, give a thumbs-up or say "Cool Bike!" And let's face it, when a 12 year old says you've got a cool bike, you do.
Despite the aesthetics, the design and materials are much heavier and more unwieldy than necessary - nice and strong for high winds, but fairly impractical otherwise - so I'll probably try working with nylon and tent materials next. Awfully tempting though to take this up to the high desert and see what it does in a really big wind.