I just bot a B&D Grass Hog trimmer. By no means trying to promote B&D products, but the automatic line feed is lotsa less messing around. I don't see any negative comments in the Usenet archives about the thing.
I bot the corded, more powerful, model partly because I figure the longer string gives the automatic line feed a better signal (the centrifical/centrifugal force is greater) so the feeding mechanism might be more reliable.
I don't understand how the device uses more string unless the length is let out too far. Apparently it hardly gets to the cutoff blade. I guess the problem could be caused by continuously slapping the line against something that repeatedly brakes it. A small price to pay for not having to mess with it. I think bumping is for the birds.
Assuming the thing works well as the first trial, this automatic line feed thing is the greatest IMO. I am going to make a spool holder for my electric drill (on very low speed) to neatly wind new string.