Pros: Easy and quick to use, lets you vacuum seal in inexpensive mason jars
Cons: I haven't found a single negative yet
I absolutely love this attachment to my Foodsaver vacuum sealer machine! I had my vacuum sealer for a few years, and always used the bags without ever trying out the Wide Mouth Jar Sealer attachment that came with my model, because I didn't have any of those mason jars. I am a total organizational freak, however, and one day decided to buy some mason jars and re-organize my cabinets. I bought these huge half gallon size mason jars, as well as some quart size jars.
It ended up being so quick and easy to do, that I sealed up EVERYTHING in my cabinets that day. For instance, I used the attachment to seal my breadcrumbs, coffee, tea, baking soda, cocoa powder, yeast, cornmeal, hot chocolate, chocolate chips, nuts, raisins, oatmeal, wheat germ, rice, pasta, and more. I have the big Foodsaver canisters for items that I keep larger quantities of, like sugar and flour, but just about everything else is in the wide mouth mason jars now. The half gallon size is very large and can really hold a lot.
I also used the jars for sealing grated cheese in the regrigerator. Unlike in the bags, when you use the mason jar the cheese doesn't get all squished together from being vacuum sealed. It keeps it totally fresh for a really long time and it doesn't get dried out either. Also, when I found blueberries on sale, I bought a bunch of packages and was able to seal them in a mason jar and keep them fresh in the fridge until I find the time to use them.
The Wide Mouth Jar Sealer attachment is also very simple to use. You just plug one end of the included tube into the attachment and the other end into the FoodSaver. Then you place the metal lid on the mason jar without the band, and put the attachment over the lid. Then you turn on the Foodsaver just like when you use bags to seal with, and a few seconds later when the light turns off on the machine, the jar is totally vacuum sealed. To open the jar later you can just use the edge of a spoon to break the seal. You can re-use the lids until they become worn, but a new package of 12 wide mouth mason jar lids is only about $2 when they do get worn out. You don't need the bands.
The only thing to make sure that you do understand, is that vacuum sealing in the mason jars is NOT the same as canning. Perishable items do need to be refrigerated, but they will last sometimes 10 times longer because of having the air removed.