@Timbur It is very easy to use. Fill the bottle to the line. Screw bottle onto the machine. Hold the lever down twice and you have sparkling water.
It works best with chilled water, we have found tap water tastes fine when sparkled. We did start by using filtered water, but really couldn’t taste the difference. The thread connection for the water bottles is the same as the soda stream bottles, so you can use those if you want.
Find the best way to buy the gas that suits you. SodaStream and CO2you both have schemes that work out at £10 per co2 bottle. Both these schemes are cheaper and easier than buying it from a shop and then taking the empty bottle back for a refund. I would guess that one co2 bottle will gas 40-50 bottles of water, although never really counted. If you can justify the initial outlay then you can decant CO2 from a larger cylinder which works out much cheaper.