I know of it, and have a spool of it, I haven't used it for fishing, but "tested" the invisibility in a swimming pool, it seemed invisible, looked as if I had a levitating weight in the pool.
It's made of flourocarbon (I think...), the stuff soem fly-guys use. Pure flourocarbon is expensive and fairly stiff, but the Vanish line has another coating on it, yet costs similar to other lines (at least where I got my spool from). I haven't heard the best "reviews" about it. It comes in big enough spools to fill an entire reel, but I haven't heard great things about it, stuff about it fraying and breaking, but I honestly don't really know.
All I know is that if it's as good as its adversised to be, why does anyone use anything else ? An invisible tough line would be a fisherman's dream.