the soda stream gets you with the co2 fills. See their bottle has a propriatery needle valve thing and you can only fill them through their 'outlets'. That small 2.5 lb bottle or whatever size it is will cost you as much to fill as a large 15lb bottle would.
Just like they will sell you a 6 color cartridge super duper printer for 99 bucks but when you buy a 6 pack of new inks it's cheaper to just buy an entire new printer... they hit you on the refills.
Here's a trick though, you can use the caps, I think you can get them from beer brewing stores, they screw right onto one liter bottles you buy at the store... think coke pepsi etc, and just use a regular co2 container regulated down to say 30 psi or so to carbonate your own stuff for a fraction of the cost.
Being an instrument and controls type of guy, I machined myself up a thingamajig to carbonate sodas this very way using any size bottle. Oddly enough I havent used it in quite some years now, I honestly really don't drink much soda at all. You can carbonate anything th ough, fruit juices work well... well orange juice. not too well... but apple juice grape juice does good.
a final say so. on the nitrous thing. yes it is laughing gas yes you get a wicked but short lived buzz but it can be deadly, also I believe it will leach B12 from your system not to mention give you a headache.... for those thinking of cracking a wippit from a bottle, be VERY careful.,..... now that I am older and a bit more less adventuresome Id suggest against it. once you pass out, it's all over.
Aaron