handig / nuttig "That is useful!" "Dat is handig!"

As Wik says, I guess that most often "handig" (handy) or "nuttig" (having utility) would be right. Sometimes, the meaning of "useful" is close to "usable." For example, when talking about an older smartphone or computer: "It's still useful." - "Hij is nog steeds bruikbaar."

just like the other people answered "nuttig" or "handig" or "bruikbaar" It depends on the maining of the sentence