We've had ours since 2008.
We are still on the 2nd CO2 tank that came with the original purchase.
You need to start with super cold water and bottles. We would refrigerate the bottles for a day or toss them in the freezer for 20 minutes to get them cold enough to hold the CO2. Always put the syrup in after you carbonate the water.
It's a neat idea, but the syrups from SS are bad, in my opinion. Until recently, they all had aspartame in them, and I am super sensitive to it (headaches every single time). We have not tried the stuff with "real sugar" in it from SS.
We were buying syrups from here:
http://pittsburghsodapop.com/
Love their syrup, but it is not budget friendly. You use more of their stuff (than the stuff from SS) because it's not concentrated. I love their Golden Cream Soda and Vanilla Rootbeer; have to mix the lemon with something because it is tart (I like it with the lime). Steel city cola is hard core cola flavor.
We didn't save any money. Just had some fun with it, for a few months.