A (very) quick look over the FLARM website shows it'll receive Mode C/S/ADS-B and FLARM system transmissions and display them on an external source.
I have a few hours experience using PowerFlarm in a tow plane in Canada.
It works well, but bear in mind that PowerFlarm only displays Mode C data if the other aircraft is being interrogated by SSR.
It displays an estimated range ring, based on the Mode C signal strength and also displays the altitude differential in hundreds of feet. There is no azimuth information, but if I see a range ring shrinking in size and an altitude display of +- 200', I start searching very intently.
There is also an issue with blind spots, depending on the antenna installation.