I use a Zaon XRX and swear by it but it does have it's limitations
Firstly of course it relys on other aircraft having their transponder on and it amazing how many GA aircraft you see in TM zones that don't and i'm not talking specials but cessna 172's etc
secondly it does pick up yourself when you alter course and sees you as a threat, that can get a little annoying but easily dismissed as it appears very close out of the blue......
......which means it is of limited use in the circuit. you just get too many warnings and convince yourself you have another or multiple aircraft within feet. It actually freaks me out.
It is fantastic though approaching an airport and transit lanes, high use areas.