Here is concise version of the seller’s (true) claims:

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These silicone sleeves ($9+) [really a “jacket,” because it fits over the bottom, not just the sides] for quart Mason jars add grip and padding, and they also insulate. These fit both regular and wide mouth jars. They protect your glass jar from breakage and your hand from cold or heat. Made of food grade silicone rubber, these sleeves are dishwasher, freezer, and microwave safe! Use a different color for each family member. The quart sleeve is about 4″ in diameter and 4-5/8″ in height.  There is a 1″ hole on the bottom of the sleeve, to help with removing them.

The hole also lets trapped air escape when putting the jacket on. (The jacket is vaguely squarish, so its corners should be aligned with the vague corners on the jar.) The coolest part of the jacket after microwaving is its bottom, because there’s a dimple in the bottom of the jar, so if the sides are really hot I slip two pinkies sideways all the way under there and let them bear nearly of the weight.
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There’s no need for a coaster under one of them, because there’s less condensation and what there is doesn’t flow down easily. I’ve found that some of the pink and yellow jackets fit loosely, so avoid those colors.

Their main virtue to me is that they make mason jars more usable, allowing me to avoid possibly unhealthy plastic containers. This is as yet a low-volume item and priced accordingly: $15 for a two-pack.

There are two cheaper, higher-volume competitors, but they aren’t good buys for anyone intending to use them to hold hot contents: Jar-z sells a jacket for $6 each, But it has these comparative weaknesses: It has a sewn seam. (The MJL is seamless, and it also has a projecting rim around its upper edge, which makes slippage less likely.) It is only half as thick, so it doesn’t insulate or protect as well. It is made of neoprene rather than silicone, so it emits an unpleasant rubbery odor, especially when heated. Jar Jackets sells a $9.75 silicone sleeve with cutouts (so one can see the level too the contents. But it has these comparative weaknesses: The cutouts allow hot glass to be felt. It isn as thick as the MLS version. The bottom lacks a hole. A couple of reviewers complained that the bottom isn’t flat, making the jar wobbly and tippy. All of these can be microwaved with an all-plastic lid in place. Two of them will contain most of the glass shards if dropped and broken.

The maker of the jacket under review, MJL (Mason Jar Lifestyle), also sells a $14 two-pack that fits 24-oz., narrow style,mason jars (3″ diameter below the mouth instead of 4″ diameter).

-- Roger Knights 10/22/18