Translator's notes

I have always heard that a good translation is one that doesn't seem that is a translation. However, sometimes the translator subtly appears when he believes that the reader will need more information than that the original gives. I speak of the translator's notes.

Some people think that translators should not use footnotes because they interrupt the reading. Or even that the translator who use it, hasn't done his job well because he hasn't been able to translate something and so he resorted to this note. Obviously, not all footnotes are necessary, but in some cases I think they are very useful, for the fact that the reader can ignore some concepts (usually by cultural differences). And if we have a curious reader, he may also interrupt the reading to understand and discover about the idea. Or in the worst case, the reader simply won't understand it.

It is also not good the abuse of footnotes, at least in the case of literary translation. In this case, we have a pleasure reading in which the reader is introduced to the world created by the author and a footnote would take him away from this world. However, in other texts (educational, scientific...), it is more common to use footnotes, which you can refer, for example, to reference sources.

As readers, what do you think about translator's notes? Do you think they are useful or are a nuisance?

And as translators, do you use it if you think they are necessary or do you believe that they are a sign of incompetence?