Yah, I've had mixed success with them as a whole. I made my first one & it worked like a champ. Lost the schematic to that one years ago.
The little bitty ones that just hang on the outlet don't seem to work very well. I'm thinking that you just need to get some power out of the thing for it to be effective and those little speakers just don't seem to do the trick. The one I keep under the RV does actually seem to work though (at least I haven't seen any signs of damage after the $800 repair bill caused by wire chewers).
The ones that advertise to feed ultrasonics back into the household wiring don't seem to work either.