I have one and use it a lot. However like people said their flavors are hit or miss. I buy powdered mixes at the grocery store and make carbonated lemonade, carbonated fruit punch, etc. which is very good.
The CO2 refills are about $15 which is good for about 60 liters.
With the powdered mixes I buy it at the store on sale it costs me an additional $.20 a liter but if you use their soda flavors it's an additional $.40 a liter.
More breakdown here
http://www.sodastreamfan.com/is-sodastream-worth-it/