FWIW, Argy now runs with one of these after one too many incidents with overly "friendly" off-lead dogs (don't get me started...) in Kings Parks. He reckons it works well - dog stops and says "WTF?" rather than carrying on to knock you over.
It has kept them at bay, I haven't had issues with them coming within a few metres of me since... but I wouldn't bet my life on it if the dog was rabid (or whatever).
It was certainly a pretty cheap investment for what it could achieve.