It's "bag in ball" so you have pressure on the PET squeezing the bag and pushing the beer out. Still seems a whole lot safer than regular kegs.
Re environment, here at least they have a recycling program for this stuff. You just include it with your other plastic recycling. Don't forget that while those steel kegs are multiple use, transportation costs (i.e., fuel consumption) are much higher just because there's more weight.
Re beer going flat, nearly all the bars I patronize use KeyKeg, and it's what Mikkeller, BrewDog and Nogne 0 use, so I don't think that's an issue.