Hi Glenn,
I have SodaStream on my 54l and it lasts me about a month. A replacement canister is £7.99 from Robert Dyas.
I like the compactness of the set up, and use the CO2 Art adapter which is very good - far better than the CO2 Supermarket option I started with, which often leaked whilst changing the canister (lost an entire canister once - gutted).
I'd say its a good introduction to CO2 injection - midway between the mini brand-name set ups you buy and an FE. I'm happy where I am..
However, I'm about to upgrade my tank to a 80l-ish TMC Signature, so expecting to get about 3 weeks out of one.. (that's a bit of a game changer, so I might have to revisit the idea)
Hope this helps.