I remember reading a topic about them awhile ago on another board and nobody thought they were any good. I'd do alot of research if I were you. They're too high tech for me, ya gotta draw the line somewhere.
I talked with a guy who had one a while back. He said its work to calibrate them. It would beep at rabbits, etc. He said you could get used to it and tell what was big too. It would be neat if they could demo it.