I like me some carbonated water personally, and tap water in Germany is super clean and tastes nice almost everywhere. However, I've heard of some people before that they dislike Sodastreams for some reason or another. What gives?
Carbonated water is slightly more acidic than still, so just be aware of your teeth and guts if you're drinking heaps of it. Some people take issue with supporting an Israeli owned company, others don't. There's always other brands if that suits you, I know aldi does one called soda king that's interchangeable with Sodastream.
I have bought mine a couple years ago already and have since not used proprietary CO2-tanks from Sodastream themselves as, as you've said, most supermarkets here carry their own brand of CO2-tanks. I usually stick to Lidl, probably something like Soda King or something