I had one of the bottle openers (for lack of a better description shaped somewhat like a yield sign) that occasionally chipped bottles. It contacted the cap at three points. After I chipped the first two bottles, it went into the trash.
But speaking of the super agata cappers, I ordered one about three months ago. It would barely seal the bottles, and I was afraid that three volumes of CO2 would blow the cap. I returned it to Great Fermentations, they agreed with me that it was defective, and refunded my money.