So I was watching The Spiderwick Chronicles, having previously leafed through some of the books (ooh, ahh, Tony DiTerlizzi art) and (a) it's a modern fantasy out in the country that still includes a fair bit of modernity through the simple use of cars and (b) it features a bit where one of the heroes has to fight invisible goblins using a seeing stone. This a few days after watching Coraline where the film has one of those as well, albeit not in a neat little steampunk eyepatch contraption.
Invisibility and variable visibility is a good visual effect, of course, and using it in the largely verbal medium of RPGs rather lessens the impact. But it can certainly add to a mystery, and as seen here can definitely add to a fight scene.
And of course, in the right circumstances, having the characters having to pass around a seeing stone, or use one in public = fun.