This is a review of the Ball Regular Mouth Jar Storage Caps that I purchased from Amazon. I like that I got 8 so that I can use them for lots of different purposes at the same time.I got these because I have a lot of canning jars that I like to store stuff that I make in. These storage caps turn a regular jar into a perfect way to store food. I never thought they would be air tight so I don't have that complaint. I can take and put a lid on homemade yogurt, smoothies, fresh salads and lots of other left overs. I like to cold brew tea with these storage lids too. Also I use a lot of different sweeteners that come in smaller bags that are a pain to always be opening. I cut the labels of of those and taped them to the jars. It is much easier this way. I put the tea bags in the the mason jar full of water before bed and I have tea in the morning. It is my favorite way to drink my oolong tea and green tea as it is not bitter. They are not for canning, but the description doesn't say that they are for canning. I have all size of jars - pint, quart and 1/2 gallon that these work on. For me these were a better price on Amazon than they were in the regular stores near me.

Mason jars aren't just for canning and freezing. They are great for storing foods, without the plasticizers in zipper bags and burpable bowls. The canning lids, however, are intended for reuse, but using steel closures results in rust problems, and using wax paper with the rings is somewhat fragile.
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If you;re storing surplus nail from a construction project - nail boxes are pretty delicate - using Miracle Whip can lids is acceptable, but in the kitchen or in the office, these caps are attractive and reusable.

I don't worry about plasticizers leeching, because I don't store mason jars of leftover in any position but erect. If your habits are different, wax paper can be used with these, the lid protecting the wax paper instead of leaving it vulnerable as a lid rind would not.

These lids do NOT provide a seal.

In case you are unaware, mayo, Miracle Whip and other brands of salad dressing have the same "neck finish" as Ball, Kerr and other brands of mason jars. Youo aren't supposed to reuse food jars as canning jars because the glass is thinner, and not tempered the same as a real canning jar, and they often break when you can in them or you freeze in them. They work perfectly fine for refrigerating leftovers, or for storing macaroni or pancake flour so that it doesn't ger infested by insects.

I notice in some comments that people, who do like them, say these lids leak. For those who aren't familiar with this, many of us use them with the metal disc part of the mason jar lid inside. Then they are completely air and water tight. So one might ask, why get them at all if you are using a part of the original lid that rusts. I use them because 1) they stack much better, which is especially an issue with the regular sized lids but even with the wide mouth lids they just don't slide around like with the metal lids, 2) they don't rust so they screw on and off easier over time and 3) they are way prettier.
I still use the original 2 piece metal lids for fermentation. I find the plastic ones, when used for a one week vegetable fermentation, smell like the fermented veggies so bad that it's tough to clean them, and sometimes I can't get the smell out of them at all, even when i have tried soaking in vinegar. There might be a solution I don't know about, and I don't know if leaving the smell in (like for using with the next fermentation) is unhealthy or not. I wouldn't use that lid for other kinds of food storage at any rate.
But I like the plastic lids for everything other than fermentation. With the metal lids including the flat disc used inside the plastic lid, I use a mesh steal wool scourer, which easily cleans off the rust surprisingly well, leaving nice shiny metal.

These lids fit perfectly on ball mason jars with regular mouth openings. They are a good replacement to the screw on metal tops, meant for canning food, and will not rust or tarnish like the metal ones with liquid storage. I am using some homemade mouthwash and needed a non metallic storage lid (water will rust the metal lids, and baking soda and salt just speed up the reaction) for the mason jar. These lids are a perfect solution, but they are not without some design flaws. The plastic is not as higher quality or thickness as I first assumed, which is not such a big deal, but for the price it is a tad bit high. Second, they do leak as others have mentioned before. Now, I expect a cap without any type of silicon or rubber seal to leak to an extent, but these, these leak pretty bad. As soon as you tip or shake the jar, it leaks. A quick shake or tip should not warrant much leakage, but these do unfortunately. Again, not a big deal for just storage, but the price is certainly questionable with these cons for the product. In the end the product does what it's intent is, and that is to provide a cap for regular mouth mason jars. They do screw on just as you expect, and remain on firmly. So if you plan on just storing food, or even liquid, just keep in mind about the leakage, and whether or not these will be a good choice for your needs.

I am often using jars to store food items in fridge or freezer. These lids make it easy. Not sure if they are leak proof, but the jars are kept upright so not a problem..