The Dynon integrated ADSB - IN unit displays a filtered target set (within about 5nm and +- 1000ft) on the map page of my system and an aural and visual warning if its algorithm thinks a conflict is possible. While I haven't kept exact statistics, I reckon I visually spot about half to two thirds of these targets AFTER they are painted - the ones I miss seem to usually be helos moving low and slow or fast things at the edge of visual range.
I treat it as another aid to situational awareness along with radio. Its greatest value to me seems to be within about 5 - 7 nm of the circuit as aircraft manouver to join or depart, in the circuit itself its unnecessary.
What I can't guard against is a purist with no radio or transponder - unalerted "see and avoid" is a mugs game.
I fail to understand why anyone would be flying without some form of radio and or electronic conspicuity device these days.