As mentioned earlier, some of the soda flavors are retched. Rootbeer, their version of diet coke (although their version of coke zero isn't bad), and a couple of the energy style drinks are just awful.
As far as the CO2, that to me was the reason that I just don't use it that much. Cartridges are like $40 a pop or something. If you like having a decent amount of fizz in your drink, then you aren't going to get 40 bottles full with that tiny cartridge they use for the soda stream. More like 20, which is what was typical for my wife and me.
Paintball fields do "sometimes" fill them up for a charge. Some fields won't because it is not a "paintball" item. As far as taking them back to stores to exchange, I haven't had a store do that for me yet. At least not any of the walmarts I've tried. Then again most walmart employees wouldn't know how to find their own assholes with both hands, a map, gps device, and bear grylls.
So it's cool in a way, and has a novelty appeal, but the CO2 cartridges are a bit of a pain.