The wife and I just got one of these, and so far we like it. We drink a shit ton of seltzer. While I don't think it saves us much if any money, I do like the fact that we don't have to recycle 10 2 liter bottles every 2 weeks. I also likke the fact that we are not contributing the energy drain and other problems associated with the shipment or large quantites of water.
As for the CO2 refills, check your local BJ's. The one in Portsmouth stocks the large refills for ~15 bucks, which is still a ripoff but not too terrible.
I also read that someone has developed an adapter that will allow the use of any CO2 canister with the sodastream. EDIT: Here is the link
http://www.co2doctor.com/