If you get one, get the paintball tank to sodastream adapter. It pays for itself after two paintball tank fills. If you have to buy the paintball tanks, it takes six fills.
Amy bought me one for Valentine's day, and I use it almost every day. I save about $25-30 a month on soda now, and I have stopped buying perier because I can just make the seltezer water myself. To get the same mouth-feel of perier, add 1/2 teaspoon of baking soda to the 1l bottle, before you carbonate.
