I've never used one. I thought they were cool too until I saw the price...
Then I realized I don't see the point in these at all. Growlers are meant to be drunk fresh in 1 sitting so having the ability to save (up to) 4 beers to drink later doens't make sense. Plus, for the price, you can buy several stainless steel 1/2 gallon growlers that will keep the beer cold all day long. Or they have little kegs that take those c02 cartridges.