We have black and white soda stream model, I think it was one of the lower end ones from big W.
We get the the refills from bigW too, they always have stock.

We were buying soda water for about 90c a L at coles before we got it and I think DH did the sums and the machine paid for itself in only a few months.

We have 4 1L bottles (2 for me 2 for DH). When we finish one we refill the other with water so it can get cold by the time we need it. Once its cold we make the soda water then usually add syrup right away. DH uses the soda stream vanilla cola. I use just a little bit of cottees natural raspberry- or I leave it as soda water and sometimes add a dash of juice to the glass.

Its seriously the best