I don’t have a soda stream, but I do have a carbonation cap for my 1/8 barrel homebrew kegs, and have used it to make “Italian soda” (carbonated water to which you add flavor syrup/puree in the glass).

Basically the same thing, just a lot bigger. I can’t fathom a reason to not want to use it if you like sparkling drinks, it’s a lot cheaper than buying soda water or whatever. Especially if you aren’t buying single use co2 charges and throwing them away after. I guess that would be my only real concern, personally. I hate those things. Such a small capacity makes them very wasteful, even if they technically/eventually get reused.