It's the one you don't see
I had an interesting experience with ADSB. The RV is ADSB out and I was using a Stratus II / iPad (Foreflight) for ADSB in. The traffic displays were very good and all looked well.
Later, I had the chance to compare that to an ATC radar plot - there was traffic that had not showed on the ADSB. The traffic was putting out mode C only but there was some problem with the altitude data so it was defaulting to "000" for altitude on the radar display; essentially, sea level. ATC was not talking to him.
I'm at 3700 - so it's outside the puck even though ground elevation was about 1000 MSL (that's the prevailing conjecture as to why it didn't display). So, the traffic really could have been at my altitude, but in this situation, it wasn't going to display for me.
As helpful as the traffic info is, I think there might still be something to be said for looking out the window since this high-priced video game is not picking everything up.
RV7A (N7101) - Flying 10/2008