Piwoslaw, I hope you are still reading this.
I bought the cheapest DS18B20 on Ebay that I could find.
And I used the OneWire library with the example for the DS18B20, so I'm sure that the checksum is tested.
I tried boiling water (should be 100 degrees Celsius) and read 98.8
I tried ice-water (should be 0 degrees Celsius) and read 0.9
My conditions are not optimal. I think that my wires took away some of the heat and cold. I soldered wires very close to the black plastic.
So I can say that the DS18B20 is very accurate.