I have to disagree with the previous reviewer. I just bought this sail and used it in light wind (8 to 10 mph), and it worked great. The large size captures a lot of wind and I could feel the boat moving at a good clip. Going downwind is a no brainer. It can even easily go cross wind. You would have to change the angle of the sail in relation to the wind - just like sailing a boat. I am still experimenting going slightly upwind, which I think might be doable on a slightly breezer day.
The quality of construction seems reasonable, although there are a few wear points that I think will break. For example, the rubber stopper shows wear already after my first use. I can see the bungee cord wearing out where you fold the sail for storage. Good thing it's got a 3 year warranty.
All in all, it seems like a really fun sail and I can't wait to go out on a windier day.