I regularly use the Dark Earth one, and I'm pretty happy with the results. Pasting it on and behind the tracks of a tank is a simply way to get an effective 'travelled' look. You can also draw track markings into it, should you want it for a dirt path or something. It's pretty expensive to cover a base with it, but I think the end result is good. It can also be watered down if nessecary, which creates a much smoother effect.