I've got the Zaon MRX. It isn't snake oil. It works well. It provides minimal information (relative height and distance) so does not encourage an eyes in mentality. It is tiny and the batteries last a fair time.
They are passive receivers only - they eaves drop on transponder conversations between aircraft and ground stations. The distance is calculated based on power output assumptions. These assumptions are used to identify the host aircraft transponder and separate from others. Occasionally the host's transponder may be confused with another aircraft's and generate spurious same height 0.1nm warnings as I had in IMC on approach to Exeter after being warned about near by corp jet. Was not happy with that, possibly bad placement by me. Despite that I think this unit is useful for IMC. If you are like me and reasonably diligent, there is less value in VMC and I rarely use it as such.
I recall I paid < £300 for mine.