We have one, like it a lot and use it pretty often. I bought it relatively cheaply on Craigslist.
Strangely, it's been close to 10 months, and we still haven't had to re-fill the C02 cartridge, I think this is because we lucked out and got a model with the
larger tank which I understand they may not make anymore.
If you are looking purely for a cost savings over buying cola, you won't necessarily find it. If you only buy regular priced name brand cola, not on sale, then the soda stream option might be cheaper. But if you buy generic cola, or just in bulk and on sale, I don't think the soda stream is cheaper after you factor in the unit, buying syrup and the cost of refills.
Now for making plain carbonated water, it's great, and that is what we mainly do. I like to drink my soda water with a lemon and occasionally some mint.
You might be able to use it to wean off of sugary drinks by diluting the ratios or adding more soda water to your cola, but YMMV.
For other users, I'm wondering how much I will continue to like the sodastream after I have to pay for my first re-fill. Who has experimented with alternate refill methods, and have you found they work well?