I use mine all of the time. The fish either must be close enough to the xducer, or it can tell the difference in the murky water. If you listen to my Youtube video, you can hear the fish alarm going off every ten seconds or so on that day we had so many fish. I am more concerned about finding drop offs and channels while dipping than actually "seeing" fish, but it does register fish anyway.
It's a five year old Lowrance X70A. I suppose any good quality (higher power) model would work though. I like the fish alarm feature though, so I know there are fish around while I'm sweeping.
GTK, thanks. I've been in the market for a system for a while and have been leaning towards a Lowrance unit as it ties in nicely with my motor for monitoring.