It takes 93 weeks or almost two years to pay back the investment. If you only drink a liter of sparkling water every two weeks, it would take over seven years! If you normally buy 2 liter bottles of water from Safeway, it will take much longer.
So, is a SodaStream a terrible investment? Well, yeah. If it causes you to drink more sparkling water instead of tap water, it’s a luxury purchase and in no way an investment. But that’s okay, most of the things we buy aren’t investments.
It's a good read, written by someone that is clearly a fan of the product.