Seconded. I drink an obscene quantity of self bubbled water. They do do sneak things like change the bottle shape between charger types and formats to force new buys.
I did the numbers and I think you break even after about 60L of soda water, if your machine costs $70. After that each L of soda water costs you about 33c. This all provided that the machine doesn't break and you don't buy additional soda bottles.
Who doesn't have a Sodastream these days? Pretty much everyone I know under 50 has one. I really cringe paying so much for carbonation, plus I like my nice glass limited edition Sodastream bottles so much more than plastic.