I guess this is an example...
Zaon Portable Collision Avoidance System - PCAS XRX Onyx - MyPilotStore.com
So, answering the questions:
1) Do you know what it is?
See URL above - I didn't but I now do
2) Do you have it?
Nope
3) Would you buy/fit it?
No
4) If so, why, if not, why not.
No panel space, no power.
Most importantly though I presume it requires everybody else to also be equipped with a similar device for it to work (e.g. like Mode-S, FLARM or TCAS). Unless there's complete interoperability between the various technologies then there's a great risk that it won't spot the other guy (particularly if they don't have any device on board).