ItalianJon,
Long time no see, how are you?
To answer your question I've had the Zaon MRX. The past tense due to being in another part of the world now. 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 it from others. Occasionally the host's transponder may not be powerful enough and may be confused as another aircraft 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, but once you are in a radar environment I think you are best served by switching it off. If you are like me and reasonably diligent, I think there is less value in VMC and I rarely used it as such.
Hope that helps.