My understanding is, Superman doesn't have any "special" weakness to magic; he's simply vulnerable to it, like anyone, presumably because magic operates on a whole other level, transcending (comic book) physics. However, on occasion, Kal has shown formidable resistance.
It seems to vary, depending on the needs of a given story.
Shinier than a speeding bullet.
Gender: Male Location: The Fortress of Solitude in Venus
Superman is not invulnerable to magic.
He is just as vulnerable as any one else.
Though this is also inconsistent in comics.
Let me give a couple of examples
In Invasion he got "affected" by magic from a very powerful voodoo priest and a scan with out context will indicate that Superman just got defeated by deadly voodoo magic designed to kill him.
Bear in mind that the Voodoo Priest on panel can kill with his deadly voodoo magic.
But if you read the comic, you will realize that before the voodoo priest strikes he mention something about Superman's weakened mental state because of his mental break down followed by the execution of the pocket verse kryptonians AND the fact that he just got blasted by a Tannagarian cannon capable of disentigrate a small ship.
So out of context it seem that Superman just got defeated by deadly Voodoo magic, but with context you realize that the "effectiveness" of the deadly voodoo magic was only partial and due to extenuating circumstances that allowed the Voodoo priest to KO Superman.
1.- Deadly Voodoo magic, confirmed to be deadly on panel to a regular human
2.- Weakened Mental state (that lower Superman defenses)
3.- Blasted by a Tannagarian Cannon that can desintegrate a small ship
So being as "vulnerable" to magic as anyone else will depend on the writer.
As far as I remember the only time that Superman showed a "weakness" to magic was when he got shot with a Voodoo bone bullet as Plastic took the same bullet but it did not affected him and IIRC it was explained that Plas got an specific advantage because of his ability of changing his molecules or something like that.
He has also survived a magical blast that according to narration will "rip a tank in two"
in short Magic is not a weakness but a vulnerability IF you have enough magical juice, IMO
Superman has no special resistance to magic. He can be cut by a magic knife but it isn't like Kryptonite, which makes him weaker when exposed to it. In that sense, his strength and durability makes it possible for him to tank vast magic attacks that would instantkill humans or even some low heralds. He feels it though and it hurts.
Kryptonite is a 'weakness'. That's why Superman's powers wane just by being within close proximity of the stuff.
That said, if magic were truly a 'weakness' for Superman, he wouldn't be able to so much as touch magical items, or be around characters whom are actively utilizing magic, without weakening substantially like he does when exposed to Kryptonite-- but this is obviously not the case, as Supes can be around magic all day long without suffering any ill-effects. However, when magic is used against Supes, it has the same effect that it would on any other person(human or super-human alike.)
So those who call it a 'vulnerability' are correct.
Superman survived recently and blast from Lexus, Devourer of Life which would have slain a "lesser herald" iirc. His resistance against magic is nonexistant compared to his invulnerability against energy or normal attacks, that's why it hurt. Still if it would be a real weakness he would have been dead.