I bought one last month. This is to stop me buying plastic bottles, and it works. I just wanted the fizz more than anything as that helps clear away the fuzz. Paid about £50 online, includes one CO2 cylinder, you can then exchange this for a refilled one at Argos, for £8, so no great distance for many people.
The flavour concentrate things are pretty vile, if the samples they included are typical, as they are all 'diet' ie taste of synthetics. Maybe the real juice ones are better. With all of them, you add the 'flavour' to the bottle after making the fizz. Nothing to stop you making your own. I find it improves the taste of tap water, I guess by slightly acidifying it as the CO2 dissolves to make carbonic acid.
I had tried before to use an old soda syphon as on a 1960s sideboard, with little individual metal gas cartons, but it leaked and sprayed water everywhere, so I was pleased to find it's all updated. The new device is a bit big, stands about 2 foot tall, because that's the size of the gas cylinder - but that makes it all last much longer. I dont know yet what the cost will work out to, but it saves dragging baskets full of green bottles back from tescos and then adding to the plastic mountain in the sea. (I have my doubts about the efficiency of recycling...)
I got this deal. - cheaper than Argos.
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