Its worth it, really.
The taste is vaguely like less-sugared cola, but the bubbles/softness of wheat beer.
Its has extremely low alcohol amount, like alcohol-free beer or kefir. Its suitable for children.
Just make sure there are no color favorings and stuff.

Or you can make it yourself fairly easy:

You need:
- rye bread (or any sort of black/dark bread some amount of rye), 100gr of bread results in 1-2L of kwas, 250gr / 5L kwas.
- 5L+ bottle(s) or similar - but it should have a cap, that can be closed/sealed air-tight
- package of yeast (approx 20gr / 5L )
- spaghetti- or similar colander with larger openings
- electric kettle (or stove)
- cooking pot
- another container or pot of similar capacity(can be from plastic)
- oven (optional, to dry up bread. microwave would also do)
- edit: 200gr sugar / 5L kwas

1. Take rye bread and slice it in small-ish pieces. Lets assume you take 200 gram for 2-3L of kwas.
2. Place it into the oven at 150-200 degrees celcius to dry fast, or you can just let it dry by itself on the air,
When its dried up:
3. Boil 3L of water (for 3 L of kwas) on the stove in cooking pot and remove it from heat source, or in kettle.
4. Place your dried bread in the boiled hot water, cover with kitchen tissue and let it stay for 5 hours (like overnight)
5. After 5 hours, place it into the colander over the other container and pour the cooking pot contents through it
6. (Its recommended to pre-heat the filtered contents to ~30 degrees celcius (not higher than 35) for this step, for yeast to work better.) Chop yeast with hand into little pieces and mix it into the filtered result. Edit: add 100gr sugar / 3L kwas.
7. Pour that into the bottle(s), which you then seal air-tight. If there are several bottles, make sure that bread/yeast-mass is about similarly distributed (steer it up with a spoon before pouring). Sealing is important, as it will prevent excessive fermentation and stop the process.
8. Place it into the dark, body-warm place (22-30 degrees celcius) for 1 - 1.5 days, then in the cooler (not freezer).
9. Enjoy!