It reduces transport waste as well, since it's far more efficient to move fluid in a pipeline than in bottles on a lorry or ship.
Exactly the point I was going to make. The environmental cost of lugging about vast quantities of water on the back of a lorry is massive. (Generally, I'm making an effort to move towards concentrated/water-free versions of products for this reason. Unfortunately, you can't yet buy bottles of concentrated fizzy water in supermarkets - science needs to pull its finger out on this one.)
I don't monitor our Sodastream use closely, but I'm guessing we get through an average 2L per day, and one CO2 canister lasts about a month - so that pretty much tallies with the claimed 60L per canister. You could easily make it stretch a lot further if you like your water less fizzy (iirc Badoit is barely even
pétillant, which might explain why it is less acidic than other fizzy water).
When the canister is empty, you take it along to Lakeland to trade it in for a full one, which costs £11.99 - so that works out at 20p/litre - although in reality it's going a bit more than that when you factor in the cost of your travel to Lakeland, and the initial purchase of the Sodastream (about £60 iirc, including 1 CO2 canister and three re-usable bottles), amortised over its lifespan - no idea how long that is. It doesn't feel like a very sturdy device, but it is mechanically very simple. That was for the basic model - there are flasher versions available, including one that you have to plug in (no idea what is the benefit of this).
We also bought a second canister to keep in reserve, so we don't have any dry spots when one runs out - that's another £23 to factor into the long-term cost.
Let's say it lasts five years before the basic unit breaks and needs replacing, that works out at an extra 2.3p per litre. And if you combine your CO2 shopping trip with another journey, that offsets that cost.
Before we got the Sodastream, we were buying fizzy mineral water from Aldi at 28p/litre, but they also do a fizzy
spring water at 17p/litre, so the Sodastream is more expensive if you're using Aldi as your yardstick. But it's a
lot cheaper than Badoit. The only downside, if you're a connoisseur of these things, is that it tastes like fizzy tap water rather than Badoit. How you feel about this will obviously depend on how you feel about your local tap water.