As good as gold.

Applied to a person, it usually praises especially dependable, well-behaved conduct. The comparison suggests someone who can be trusted to do what is expected without causing difficulty.

The phrase can also describe something as fully satisfactory or sound in condition. In that use, it conveys strong approval, though the precise emphasis—quality, reliability, or health—comes from the surrounding sentence.

Use it informally as warm praise: “The children were as good as gold” highlights excellent behaviour, while “the old machine is as good as gold” can affirm that it remains reliable. It is generally complimentary and often slightly emphatic.