I used to use Zaon PCAS (the cheap and highly transportable one). It is good, but not the be and end all. If you rent aircraft then you may have issues with false alerts. The reason is that the unit makes assumptions about attenuation of the signal to determine range and sometimes thinks the host aircraft's transponder is a threat. Once you have figured out the best placement then it should be OK or else the host transponder is weak and needs a seeing to.
No issues in my cub though, no tranny
I have to say by and large it didn't detect that many other aircraft that I wasn't aware of from R/T, in the time (albeit just a few months) I flew with it there weren't any significant threats. It was certainly reassuring when in IMC to get traffic information and then see it on the PCAS.