I don't think distant terrain would work well. There'd be tearing and cracking and all sorts of nasty artifacts.

Distant objects, on the other hand...

Every other frame is too often! Impostors really shine when you redraw at rates from from 6 to 1 FPS.

So: impostors are best used for static objects like rocks, trees or even buildings (preferrably with a depth component so they can be blended flicker-free into the scene, like they were 3d). Slowly moving creatures as well.

But! A modern impostor must be made using multi-target render, to consist of, at least, color, specular, normal, glow and depth textures so deferred lighting could be applied to it.

Also, you need a sophisticated manager that decides which objects are rendered normally and which use impostors, how much texture memory each one uses (hello, fbo space manager). And, if you think of it, lots of impostors require lots of texture space.

To summarize, the best application where impostors shine would be a dense forest.