I just recently acquired the magic weapon skill, and it seems to be bugged. I'm not exactly sure how this is supposed to work, but after casting I see no indiciation that this skill has been used. No countdown timer, no change in the visuals of my character, etc. I'm guessing this is a bug, as other skills - such as diamond skin - clearly display a countdown timer, and the visuals of my character change as well.
For what it's worth, I have a level 21 wizard, normal difficulty, am playing single player.
edit: My bad - looks my weapon does glow a little differently after casting. I do think that it would be helpful to have a timer displayed on the screen, but it look this is not a bug