I built one of the Stratux ones. Amazing thing is how many (even high level RPT aircraft) you can spot visually, with the aid of the output of the unit displayed on the EFB screen. None of those high level aircraft is a collision risk - very ‘high’ while us nobodies are very ‘low’ - but it does demonstrate that the unit picks up and provides pretty accurate information about aircraft putting out Mode C data.
But the ‘blindspot’ is that it requires ‘something’ to be interrogating the other aircraft’s transponder….