I bought that purple cushion (The Gypsy SitUpon) recently. And I love it. I am a tall person with lower back problems. This helps with both. After getting used to it, my back (and butt) don't hurt after a sewing spell anymore.
The best part is the added height. I use my machine on my dining room table, and by adding the height of the cushion, I'm finally at the right ergonomic height for using my machine. That might be part of the resolving back issue too.
I have found that I put a cloth over the nubbies though, as they tend to feel pokey after a while. Maybe I should just use that as a sign to get up and stretch ??