MArcomage

Anyone ever uses it?

I don't use it, but someone has played it against me already a few times, but very rarely.

I've had it used against me. Been a while since I've gone against an Illusion or Destruction deck, though.

I've never been much inclined to use it. I think a lot of cards severely overvalue facilities (like this one, kidnap, university, elven apprentice...)

Treacherous artifact is annoying. Kidnap is slightly overpriced. This one is too expensive. Even Curse is better. I tried to use it in one of my decks, but Treacherous artifact and even Kidnap were effective there, while Mind control was a deadweight.
And Illusion is very gem-dependent, so it's hard to play it without being screwed, when you play Illusion deck.
I'd say that this card should cost like 20-21 gems, and Kidnap should lose some cost aswell. Treacherous artifact is fine... I guess.

although on the paper - magic-- costs 13 and magic++ 12 = 25
and here on card does both.

The problem is, you need this -1 Magic as fast as you can to prevent enemy gems, as well as +1 Magic to accumulate gems faster. You can play Book of Magic fast (round 4-6), as well as Immolation, but when you are using Mind Control, you are FORCED to untie enemy hands and let him accumulate some gems or use Book of Magic, Apprentice or Restorations.
Illusion keyword is good, yes, but there are much more cheap and effective uncommon Illusions. Or Curse, if you want an omnistrike.

Maybe make it cheaper but with a certain illusion based condition?

I'd remove Illusion keyword and make its cost 18-20g.