This is an opportunity for me too to share with you my experience with bubbling tap water. I've had an ART version of the SodaStream for two months now (it's the retro one with the lever on the right side). I am just finishing my second CO2 bottle. During these two months I have been keeping statistics on the gassing performance. With me it looks like I managed to get a gassing result of 47 original bottles x 0,8 L = 37,6 L of gassed water from the first CO2 bottle included in the kit. I give 5 approx. 1-second shots per bottle because I like the bubbles to pinch my throat (maybe not the power of Żywiec Zdrój strong gas, but rather a comparison with Cisowianka carbonated). I chill my tap water in the fridge for at least 2-3 hours before carbonating (apparently cold absorbs carbon dioxide better). From the second bottle I currently have 45 0.8 L bottles on the counter. I may be able to get to 46, but the pressure is already very low. Regarding gas escaping when gassing water, this is supposedly normal. At my place, after the "shot", there is a brief hissing of gas somewhere near the water bottle holder (neck). I always wait a while (about 3 seconds) before firing another "shot". And so on for up to five shots. When the gas in the CO2 bottle runs out, the pressure starts to drop, so I hold the gas lever longer at a time, even up to 2-3 seconds.
Personally, I am happy with this invention so far. I have good water in my tap and after gassing it is much better than some Polaris from Lidl. And you don't have to lug around those 1.5-litre PET bottles from the market. I replace a spare CO2 bottle for 25 PLN in the local Leviathan and I see no difference in quality or taste compared to the original. Regards.