My first qualm with Mindslaver is that it is a bad idea overall. It is an idea, much along the lines of the indestructable mechanic, that probably appeals to children very much. For years, stupid kids made their own cards that would say something like "this creature can't die." But when something like that ends up on real cards, it's an entirely different matter...
Mindslaver has that same feel to it. Being able to control your opponent is, I believe, a horrible idea to put in a CCG.
My second qualm with Mindslaver is that WotC knew or should have known what they were doing. They already printed Word of Command over a decade back. It proved to be a bad idea. Of course, they have a habit of repeating mistakes, so that's no surprise...
My third qualm with Mindslaver is that it hoses good cards. Necropotence, for example. Um, you pay all your life. Oops, you lose. Wow, that was fun...
But against nothing in particular, it's a ten-mana Time Walk, as it has been called before.
My fourth qualm with Mindslaver is Goblin Welder. Seriously, it's not fun to play against. And I'm a fan of prison decks. Slavery isn't the same as prison...
I think that covers why I don't like Mindslaver. As to how good it is...
Well, it's generally more of a question of how good cards like Workshop, Welder, and Mana Drain are. Those are the cards that enable it. But Slavery decks are doing pretty well in tournaments, so that means the card isn't crap, to say the least...
