Incredibly good for strengthening all the surrounding muscles to knees/ankles. Fill it up and balance on one foot, you will constantly have to adjust to keep balance. My left leg has problems, and i can tell that when standing on the cushion that some muscles are underused/under-strengthed in the left leg.
Product has a large hole and plug, not a football adaptor size. I just happened to have an adaptor that fit. Plug has kept the air in for many days now, so doesn't leak.
This product is big enough to stand on with one foot, or two.