Thank you for your helpful replies.  I think the PHD would certainly be better than digging by hand (it usually takes me 30 min to dig a hole for a moderately sized schrub due to all  the rocks). But I can see how rocks can impede progress. With I back hoe you can dig the rock out. 
Does anyone know how well the augers wear?; eg. when do I need to replace the cutters.
Thanks
	
		
			
		
		
	
				
			Does anyone know how well the augers wear?; eg. when do I need to replace the cutters.
Thanks