I like onomatopoeia and find them very important in a comic. It definitely shows when someone really works on their sound words and integrates them well with the art. I usually don't read them, so I don't mind cheesy, overused words. But the good use of typography adds something special to a scene.
However, I do find it really hard to make them myself. It is hard to come up with them (especially if English is not your language). Drawing them is hard too, typography is not easy. But I keep doing it anyways because I am hoping that I will improve with time.
I only add them when I want to draw attention to whatever makes the noise. Example:
A character slowly moves his heavy leather jacket to the side, to reveal the weapon attached to his hip holster. It's an important gesture and deserves a nice little onomatopoeia (like "sssssst" maybe).
At other times, when the character does the same movement to draw his weapon before a fight, I would not add the sound word. Something else will be more important and will get the onomatopoeia instead.
Currently I come up with onomatopoeia by actually making the noise myself. What sound would I make to describe a big cloud of dust approaching very rapidly to someone, who does not speak the same language? "SWOOSH" or "FUAAAAAH" or "SHAAAAA" maybe.
An explosion could sound different depending on the type of explosion. A big slow mushroomish like explosion could be a "WHOOOM", exploding metals could add sounds like "CLANG" and "BLANG".