I'd say that "ништяк" associates with "ничего себе", which means "wow". And often the meaning is "вау ништяк". However the "wow effect" of a "ништяк" might be weak (depends on the context).
Another similar, but different meaning is "кайф", or "ништяк кайфово", which means "kaif", "high", "buzz", denoting very high excitement.
Not very often, "ништяк" can be used as a noun, where it means something blurred between a feature, an interesting thing, and a gift. It can be also in plural "ништяки". For example, "Дед мороз принёс нам кучу ништяков". "В сервис добавили много новых ништяков". "Я там тебе пару ништяков на окне оставил."
It can be also used in an adverbial form "ништячно" or "ништяково", which means "cool". But more often people will say short "ништяк". For example: "а ты ништяк поёшь" (could use adverbial form here) or "а ты поёшь ништяк".
Similarly, it can be an adjective "ништячный" or "ништяковый". Examples:
"а штаны у тебя ништяк", "а у тебя ништяк штаны", "ништяковые штаны", "ништячные штаны".