I like it. As a DM who plays about 40% of the time, I like a magic item that looks ordinary, isn't difficult to use (I mean, a use magic device check to draw on a wall? If that was true it would have kept me out of so much trouble when I was 3).
The part of this that is great is the fact that it is devastating in the hands of an inventive DM or player.
Need to read a paper you know has explosive runes? draw a blast shield and read away.
Need a Temple of doom-style trap? Now you TOO can get back at the DM for his arrow-trapped corridors.
It's kind of a limited Green Lantern ring.
