Anyone tried out these inflatible seat cushions? How do they hold up and keep from leaking air? I met a guy who has one and he swears that it helps his bad back. He also said that they sell for a lot less at medical supply stores. I've been looking for one at the med. supply stores but the sell them for almost MSRP of $150.00 he told me that he paid $50.00 for his but did not remember where he bought it online. I would like to try one for what he paid but not $150. thank you for any info. curley