Not amazing, in the end, with overgrown edges, I resorted to a $6 buck bros. saw and just cut along the sidewalk edge.  The B&D Unit couldn't cut through, kicked up a cloud of dust & grit and you had to pass over the area to be cut multiple times just to perforate (and not even cut through).  Maybe around a well maintained edge, this would keep the edge maintained, but there is no way it will cut in a new edge on an overgrown surface.  Not sure if I'll be keeping it - I'll give it one more try on the newly hand sawn edge and see if it works for maintenance.  But if you have overgrown edges along sidewalk, path, the hand saw was quick, efficient, left a clean edge and didn't over me, the operator, in a cloud of dust and grit.