Little - I was very fortunate to have a local hobbyist growing them. I have no idea where I would have gotten the plant otherwise!
AQUAMX - Cursed shrimp!
They certainly do make maintaining a separate substrate challenging. To clean the soil from the sand, I just siphon off the top layer and siphon out everything right at the glass. Then I just "top-dress" with fresh sand. The broader your physical barrier (I'm using rock), the less mixing you'll get. I've got spots where there's about an inch between the Aqua soil and the sand where I get a lot of mixing, and I have areas of about 3 inches where I get none at all! Hope this is helpful!