How often you need to use it to make it worthwhile: Every other day for eight months.
If you drink seltzer or other sparkling beverages like it's, uh, water, it makes economic (and eco) sense to purchase a machine. Let's say you finish a bottle of store-bought seltzer every two days (which is about how long it takes before the bubbles get flat): In eight months, you'll have gone through 120 bottles, which would cost about $80 (at 67 cents per liter)—aka the equivalent of your initial investment in a Sodastream. And that's just for plain, sparkling water. If you're making cola or another flavored soda, you'll save even more.