Depends on your process really. Try to get as much out of fermentation co2 as you can, I spund at 15psi, the blowoff goes into a keg full of sanitiser that's hooked to an empty keg, use the blowoff to push the sanitiser from the full keg into the empty and then from the now full keg back into a bucket and stick another spunding valve on the kegs when empty and get those pressurised and ready to go, this is also a good way to get your transfer lines santised and ready to go. If your not using the sodastream to purge your serving kegs you'll get more out of it. I have a big co2 bottle, I got the sodastream setup for a minikeg but woke up one morning to an empty co2 cylinder thanks to a leak and the sodastream kept me going until I got a replacement. They're a handy fallback if nothing else, you will get mileage out of them, how much is kinda subjective because temperature and desired co2 volume will dictate a lot of your usage. Its a good option as a jumping in point (in my opinion, others may disagree) and it has a place in your inventory even when you move to a big co2 tank and unfortunately that's not always the case with homebrew equipment