I adore my Sodastream
and use it all the time. They are expensive enough as is, so get the basic low-end model. They all do the same thing: shoot CO2 into water quite efficiently.
As far as the supplies, I bought a bunch of those and quickly burned out on them, except for the diet pink grapefruit soda flavor which I use to make a grapefruit-basil-elderflower martini. I make my own flavored syrups and we love those so much better than the soda stream syrups. I made a Dr. Pepper knock off that people went nuts over with raspberries, prunes, cherry leaves, spices, etc. You can also better control sugar content in the ones you make yourself.
The other consideration is watch sales, Bed Bath Beyond coupons are great to bring the price down. I bought mine for 50% off at local Meijer store, the box had been damaged, product was fine. They also have sales on the flavorings. I found a buy one get one offer a while back.