Interesting product offering. Shipped quickly and very well packaged. I originally bought these thinking the roller balls would be larger than on smaller bottles I acquired elsewhere (and I got a 50ml bottle too), but I guess no matter how big you go they are all one standard size.The roller-head assembly on these is a little more challenging to install. I recommend using/making a proper tool for this.As far as the seal, it seems that when you tighten the cap it sets onto the top of the roller assembly in such a way so as to make a real tight seal. That's certainly unique with this product compared to others, not to mention the glass choice.All in all I am happy with my bottles purchases from Infinity.Update: 12/13/2015I have been using these jars for essential oils plus solvent-like ingredients in my recipes for healing treatments. I cannot rely on some of the other non-steel rollerball type jars I have picked up elsewhere for longer-term storage. I have found the Infinity Jars to be quite nice/good so far.I have some friends who brought jars back with the stainless steel ball falling out, i.e., no longer being held in place. Iam using a couple ofmine as much as they are without the same issue. As long as we keep the lid on tight for transport or needed shaking there's no leakage, which is very important. In use, they flow quite a bit with the ball in place. Not a problem for my application, but may be a problem for perfume or other 'finer' applications where you don't want a strong flow. I am still buying some more jars from them.