I have one and I love it. I use it mostly when I do custom quilting on my mid arm machine. It is completely adjustable (height, angle, etc.). I have a rug in front of my machine, which makes it more secure. I think it if were rolling directly on the floor though, I wouldn't feel as secure (It would probably roll out from under me!). And yes, it stores nicely under the frame.
Here is a link to the one I have:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1