If you're a DIY type, have a little extra space and go through a lot of fizzy water, you can make your own carbonation system pretty easily. You need 4 parts:
-CO2 tank, $30-60. I bought a 10lb tank for $50, which can carbonate at least 200L of water between refills.
-CO2 regulator, $50. I bought a dual gauge draft beer dispensing regulator that screws into the tank. The gauges show the pressure coming from the tank and the pressure going to the bottle. You can adjust the latter depending on how fizzy you want your water.
-Gas line with ball lock assembly, $10. This is just the line that runs from the regulator to the bottle.
-Carbonation cap, $10. This is a stainless steel cap that screws on to any standard soda bottle and attaches to the gas line.
Disadvantages vs. Sodastream
Too big and ugly for a countertop. I store mine under the bar in my basement.
Larger upfront investment of about $120.
Need to go to a compressed gas supplier for refills (but the folks at NFC are wonderful).
CO2 tanks need to be tested every 5 years, which costs $10-20.
Advantages vs. Sodastream
Cheap refills. It costs about $20 to refill a 10lb tank. It would cost about $300 for the same amount of CO2 if you buy Sodastream cylinders at retail (14.5oz cylinders go for around $30 each).
Large tank means fewer refills. I refill mine about every 2 years.
Simple to adjust pressure.
Use with any soda bottle.
After carbonating about 100L, the DIY system is the cheaper option.