I love SFX and think they are necessary. I am pretty relaxed about sound effects when reading comics, even if I see well-used ones like 'bang' or 'knock.' However, the ones that do get me are the weirdly descriptive ones. Sound effects are the result of an action that has happened or is happening; they should not describe it literally. I wrote some musings on SFX awhile ago, but my blog is currently unavailable, so in short ...
If you have sound effects that are sentence fragments, or include an adverb, such as 'sadly sighs,' 'stands up,' or 'breathes heavy,' they will probably be read comically. I also find verbs that describe actions that make no sound (thrust, smile, touch, look) to be mostly unnecessary. Serious, beautiful, comics have made me giggle with sound effects like that -- it takes a bit to push forward with the story and try to tune out the SFX.
I also find that if your sound effect has to describe what your character is doing rather than assign sound to your comic's world, your art is not getting the point across. I love sound effects -- but they should be just that. Sounds! Don't add the non-SFX 'stops bullet' to an image of an OC stopping a bullet as well as adding another character gasping, 'He just stopped that bullet!' A little much, in my opinion! I tend to see SFX as sounds rather than read them as words, so seeing something that is supposed to be an SFX that really isn't … is a bit jarring.
Use them a lot, use them boldly, use them well. Think about the object making the sound, how long the sound lasts, what the items making the sound are made of, how important the sounds are, etc. You can show the sounds by thinking about the font or impact of the written SFX as well as what you actually write -- so don't be afraid to experiment with that. (Don't get stuck using a single font!) For example, if a sound is really loud or jarring, show it with a scratchy, bold writing. If it is a quiet, unobtrusive sound, try a little, neat font.
Here's an easy tip I read somewhere (source escapes me -- will edit if I find it!) on turning some of those old favourite SFX into more of a sound and less of a word to be read. Instead of using 'click' for example, switch out some letters, remove some, and maybe go with 'klik.' Instead of 'bump' try 'bmp.' You can make SFX sound harsher, shorter, more precise. You can also drag them out by adding vowels. So, perhaps a door creaking open might be more effective with an SFX such as kreeeeeeeee.
I think SFX are necessary. But think about what ones are important. You wouldn't write out every single sound in a noisy baseball stadium, for example, so choose the ones that matter most. Similarly, not adding a sound to a significant action can come off as weird and impact the mood or feeling of the scene. Decide which sounds are most important. A possible use for SFX-overload might be if your character is feeling overwhelmed -- in that case, write out all those sounds, press them in around your character! Or perhaps your character can't hear anything after a nearby explosion; in that case, the SFX would be absent. SFX can do a lot more than give sound to your story.
One of my favourites I have seen is 'frask,' in a comic where a character was crunching through snow. The fresh, crisp kind of snow that happens on a clear night after a warm day and it has iced over with a slight crust. 'Frask' is perfect for that sound!
SFX are fun. I pretty much just sit around making noises, trying to figure out how sounds can be written. I do like some of the old favourites (boom, bang, etc) but I tend to use my own. Even if you do use your own, be aware of well-used SFX so you can avoid misunderstandings (which might end up unintentionally hilarious). Don't be afraid to bend the rules now and again: you can give sound to soundless actions if the mood of the scene suits it. All in the best interests and feel of your comic!