I like to use a hen decoy when setting up on a bird that will have a long line of sight between himself and my position.  Usually that's 20-25 yds out in the field when hunting birds roosted on the edge of the field and will be flying down into the open field.  This has to be done in the pitch black dark... do it when it's getting a little bit light and you will end up doing more damage than good.

I think decoys actually hurt your hunting while in the woods.  They usually aren't necessary if you set up in a location where the bird has to come into range to be able to see where you are calling from, and I honestly think you end up spooking more birds trying to place a decoy in the woods.

I don't have any long straight logging roads, but they could be useful in that situation if there is no terrain feature on the road to create a visual barrier between you and the tom.

I will say this, they really only help with toms in the right mood... fired up, see the decoy, and come running.  Most wary toms will watch the decoy from a distance, and either lose interest if it doesn't move, or realize it is not a real hen and not come to investigate.