There are three major costs to Sodastream. I used the Target website to find the information I needed to evaluate this. The initial machine is about $80 (although some models go up to $200) which comes with flavor packets and a carbonation tank. There are the carbonation tanks which run $15 and carbonate up to 60 L of drinks. And finally, there are the flavor packets which run from $5 to $12 for about 12 liters of servings. So then, how much soda does one have to drink to make this economical?
Total Liters Made
$5 flavor Cost / 2L
$12 flavor Cost / 2L
Now, like I said, I am no soda connoisseur, but I am pretty sure that those prices are much higher than what I can buy in the store. So I looked. Sure enough, name brands go for $1.50 to $2.50 for 2L bottles and generics for $0.50 to $1.50 for 2L bottles at the local grocery store. But the next question would be at what consumption is this economical?
That depends on the durability of the machine. The $1.50 for generics is broken at the 480 L level. Assuming a 3 year life, this is almost 7 Two liter bottles every month (our family maybe goes through 7 in an entire year - unless I am buying them for Mentos geysers in which case I'll go through 7 in about 5 minutes, but I digress). But that is the top of the generic price level. Unfortunately, because of the cost of the carbonator and flavoring, the cost for 2L will never be less that $1.33. For name brand it looks even worse, the flavoring and carbonator make the bottom price $2.50 per liter. So I would judge this as not that much of a money saver. It is almost always going to be cheaper to pick up your soda at the store - even with ridiculously high deposit fees.
Let me alter that statement: Unless you are accustomed to drinking premium sodapop, this is not a good buy. However, if you like the flavors or like to make other things carbonated, then have at it.
About 20 years ago, I was at a party where a friend had something similar. We were using it to make carbonated eggnog. It worked sporadically at best and made a mess when not used exactly right. I have no idea whether the Sodastream has similar problems. My guess is that the fad will wear out within a year or two as machines break down and people realize sugar, water, and carbon dioxide are REALLY cheap to mix together (which is why they have specials for 2L bottles as low as 33 cents!), ergo not worth the time to do it yourself.