Personally, I find the concept of a release that tries to protect a shop from a suit based upon the shop's negligence to be offensive. In practice, the number of renters who are in position to thoroughly examine and verify all aspects of rental gear is probably pretty small, and the shop ought to be responsible for verifying the quality and quantity of gas in a tank they rent. If that weren't the case, then why would anyone have ever been upset with Gundi? After all, the issues there were a result of gas quality, and it would have been the responsibility of the divers to test that before using the tanks, right?
No piece of paper should release someone from responsibility for the results of their own negligence.