Thanks, I was hoping to hear from an actual user.
I've known people who have used these. Some have found them to be a device that can be tricky to use, others have loved it. I fall on the side of thinking that they're pretty cool. They do work in cross winds, you don't need to have the wind from directly behind you. Having said that, on this forum especially, you're going to get a lot of users who prefer the Hobie sail. That's not necessarily because they have a bias towards Hobie (although, most of us do) but it's also because the design of the sail for Hobie kayaks is a better design. There's a reason why virtually every modern day sailboat uses a sail like the ones that are used on the Hobie Kayaks, as opposed to the sail that you have mentioned. That style works better. If you don't have the option of trying a Hobie sail, the WindPaddle sail is a fun option. If you do have a Hobie kayak, you'll have more fun, and more capability with the Hobie sails.
Hope that helps a little bit.