A long time ago I thought a sawzaw type vibrating shovel would be clever. Then I thought about the weight of the unit and decided it would be useless.
I saw the Dyna Digger in a trade magazine, went online and watched a video.
I think it would be fine if you had a tree farm. For irrigation work there is no way I'm lugging that 50 lb thing around. I think a pick and a sharpshooter shovel could outdig it in trenching because the operator would be so fatigued from just manhandling it around.