If you say that someone or something is invaluable, you mean that they are extremely useful. He was an invaluable source of information. This experience proved invaluable later on. Invaluable is not the opposite of valuable.

What does it mean when you call someone invaluable?

The original (and current) meaning of invaluable is "valuable beyond estimation"; the word describes something so precious that one cannot assign a price to it. This, clearly, is the opposite of the meaning "having no value; valueless" that the word might seem to carry.

Is invaluable a good word?

Invaluable, on the other hand, means "valuable beyond estimation." Much like priceless, it describes something that is of such a great value that it cannot fairly be quantified: Great mentors have had an invaluable impact on my career success to date.