Wouldn't you save a lot of bother by simply buying cheap mineral water and mixing with that? That's what I do.
You can get a 1.5L bottle of perfectly nice fizzy water in Germany for about 20 cents if you recycle the bottle and get the deposit back.
I recently bought one for convenience reasons (after i learned about th glass caraves) and am totally happy with it - I guess as more (young) people in the bigger cities don't own a car (and only buy small quantities of food in the supermarket around the corner), more will use products like SodaStream.
Pricier than it should be perhaps, but I honestly think I have an addiction to it; I've been drinking carbonated water at home almost exclusively for years. I'm considering getting a professional carbonation setup like a bar might use.
That must be nice, but in the US (at least where I live), a 2L bottle of seltzer is 80 cents, minimum.