Also, ordering CO2 online is no big deal once you do it. They give you a pack to return the used canister(s) . If you order enough you are only dealing with canisters every 2 or 3 months. Trying out different syrups is also fun and likewise only needs to be done every month or two once you know your needs. I don't think that there is a great cost savings, but there is some savings and not lugging 6 packs or larger from grocer to trunk to house and then feeling like you have to recycle is a big plus.
I have a FIZZ. It takes the large and small CO2 canisters. The large canisters last more than twice as long as the small. To make good soda you should use refrigerator cold water. So, get a couple extra bottle and prefill and keep them chilled. Chill your syrup, too. However, I noticed that the regular cola syrup is pretty thick and doesn't mix as well when cold. The machine is easy and fun to use, but does require a little getting used to and some experimentation which is actually fun IMO. I use only diet soda and have only tried the regular cola out of curiosity because it does have fewer calories than sweetened sodas. The product is good and it will probably get better because they do have room for improvement in getting it to be a little easier to use and standardizing the finished drink. Odds are that will give them the ability to resell you on a new unit every couple years, much like Gillette and Schick.
I have had one since early January and love it and use it quite a bit. Bought one for sister as a gift and another gift for a friend. Got turned on to it by my step son who had one for about a year and half. He loves it no bottled to lug up five flights.Three kids who use it quite a bit.