I was able to get a perfect seal on eight bottles. But the other four, no matter how hard I hand cranked to the point of no more movement, still leaked. As you would expect, those four bottle are failing the carbonation squeeze test. I would still recommend this product but with reservations.
I ordered these bottles after I had started a batch and was worried that they wouldn't be delivered in time before the fermentation was complete. Luckily MrBeer was pretty prompt about shipping them out and they came just in time. The bottles served their purpose and I think it's a lot better than spending a bunch on glass bottles and a bottle capper, in my opinion. Should be drinking it from a glass anyways. My only gripe is that there wasn't enough beer from my batch to fill all the bottles!
They are great and work perfectly for the process as my boyfriend and I are new to this. I see other people talk about using other types of bottles but I think these are perfect for the first couple goes at it. I thought they came with the carbonation drops though as the last set I ordered did but these did not! Overall though they are great just like all of my other Mr. beer purchases!