Same: I ride one of the school horses with them as he's rather beligerent and can be a touch risky in experienced or nervous hands. I've never had to test the mechanism though, and the two falls I've seen from him by other people have been so clean the quick release wasn't needed.
i have these stirrups and have had them since they 1st came out and although they are heavy they are very good saftey stirrups, and if they are paired with the boots can help keep the stirrups on the ball of the foot and heels down