Overall, this is a great product, especially for the cost compared to other yoga ball on . You get the product pretty much as described. My review is mostly about the sizing.I initially purchased the "48-55cm, M" as a replacement for my wife's yoga ball chair, as it needed a 52cm. When placed into the chair and aired up, at 52cm it was very soft and not very stable (pictured up close sitting on it). Airing out up more, only made it harder to fit into the chair (pictured not fitting).Thinking perhaps 52cm was incorrect, we decided to purchase the "38-45cm, S" but that was way too small, even aired up all the way (pictured sitting in the chair divot).So we went back to the 48-55cm, M. While it was a bit too squishy, we added a folded towel to the top and that gave it a little more heft and stability that made it usable. Then we covered it all with one of her floor pieces to hold everything down and that seemed to do the job.So overall, the sizing is really the top number and it form, anything less is pretty squishy or deflated significantly and needs to be modified (if possible) to get the desired firmness.