Jump-scares: What's your opinion?

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Most things have already been said, so I won't repeat them. Maybe add this: If the player fears the jumpscare more than what's causing the jumpscare, you're doing it wrong.
It's easy to make the player afraid of being startled again, since it's an uncomfortable feeling, similar to an electric shock. But that's cheap and won't create more horror atmosphere than standing behind him with a taser would. (Yes, I'm looking at you, Dead Space )
Instead, the fear should stem from what the jumpscare implies, i.e. that there might be a monster nearby or that there are inexplicable things going on... whatever works well with the story, setting and atmosphere instead of distracting from it.

Secondly, one should keep in mind that jumpscares wear out very quickly. The player will soon start expecting them, rendering them tedious and boring after a while. So just use them often enough that you keep the player in a constant state of expecting something to happen, but fullfill that expectation rarely enough that he can't rely on it.