On a related note 50/50 soda water and fruit juice FTW.
That is what I have been doing.
I tried the sample packs included when I bought the soda stream and some of them were okay. By the way, their non-diet syrups use real sugar and Splenda. The calorie count is lower than regular store bought soda. Also, if you purchase a 2 liter bottle of Coke, or whatever, at the store and it goes flat, you can use the sodastream to carbonate it again (you will have to pour it into their bottles first).
In addition to the regular and diet syrups, they also offer those light "essence" type flavors that don't use any sweetener and give the water a slight hint of taste.
However, what I normally do is purchase, or make, fresh lemonade or get some not from concentrate grape juice. Then I mix it in about 2/3 water to 1/3 juice ratio (still experimenting) and carbonate that mixture.
The sodastream does a good job carbonating the beverages and I would recommend it but there are some downsides.
First, the 1 liter bottles aren't very big. 1 liter is maybe a little over two servings for me. If there are more than a couple of people in your house drinking the stuff you are going to need a bunch of bottles and/or you will be constantly making the stuff.
Also, there isn't a real good way to label the bottles, so you are on your own remembering what is in each one. Sure you can tell by color but if the colors are basically the same you are kinda screwed.
Overall I really like sodastream, but not as a cost saver. I like it because I can make carbonated beverages without a lot of crap in them that are too calorie heavy.
-KeithP