One of the planes at my FBO has a standalone Monroy TPAS box (can you believe it!?! in a trainer!?!)
It detects traffic very well. If an aircraft with a transponder is within about four miles, it'll give the old "traffic traffic" until you get annoyed and switch it off. That's about it. No direction or relative altitude.
Now the
Garmin GTX 330 Mode S Transponder, on the other hand, is what I'd prefer. And all bandits --err-- "other aircraft" are displayed in a plan view on a GNS 430/530 (MSRP for the setup is $15,000 to $20,000)
Did I mention you can snag a TPAS box for about 3% the cost of the Garmin setup?