In some ways, I already play with impenetrable forests. Me being the paranoid guy, I avoid Ancient Forests at all costs, taking all sorts of crazy detours and only striking at the cities I can reach without traveling through those nasty trees. When I do have to, I always send one weak, sacrificial unit forward first, ready to have my big stack run away screaming. I feel kind of silly, given the low chance, but it only takes one Treant to really ruin your day.
Anyways, I can say from experience that impenetrable trees are not all that awesome. It forces an annoying amount of movement, makes only certain cities viable targets, and inhibits general game progression. If, as I noticed one poster describing, it occurs only in pockets, then the effect would be too minor to really do anything other than cause the occasional headache. Not sure if that's really the desired effect.