Superman is susceptible to magic the same way anyone else would be. Assuming there was such a thing as magic, that is. He is no more susceptible than you or I would be, but because he's immune to so many other things we are not, magic gets used against him a lot. (All you need to create peril for Batman is to point a gun at him...with Superman it takes something special, so magic gets overused.)
Which is to say, as we discussed during JLA/Avengers, if Thor hit Superman with his magic hammer, it could hurt him where a normal hammer would merely shatter. Or if someone stabbed him with a magic sword, it would cut him...'cause it has magical properties. Or if a witch cast a spell on him, it would work. But the mere presence of magic doesn't harm Superman. I mean, he can stand next to Zatanna without fainting, for example. (Though to be honest, I'm not sure I could, especially if she were in the fishnets!

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Superman is invulnerable to all manner of natural and man-made forces, but not to things that, by merely existing at all, break the laws of nature and science. Same with a human...a bullet-proof vest won't keep you from being turned into a frog! In other words, while Superman is susceptible to magic, I don't think it's right to list it as a "weakness." If you did, you'd have to list it as a weakness for every character in the DC and Marvel Universe. Even mystical characters like Dr Fate or Dr Strange are vulnerable to the magic of other mystics.