I've flown off and on the last 3 years in various aircraft equipped with TIS (Traffic Information System) or TAS (Traffic Advisory System), but the majority of my flight time has been in aircraft without any such system. What I've learned is that there's a lot more aircraft sharing the sky with you than you'd think if you're just using your eyes. I've also seen aircraft that didn't appear on the TIS or TAS system even though they were clearly visible to the naked eye, so these systems are by no means foolproof.
Most non-pilots don't realize that without an operating transponder, small aircraft don't show up very well on ATC radar. And without a functioning transponder, an aircraft won't show up at all on a TIS, TAS, or airline TCAS (Traffic Alert and Collision Avoidance Detection) units. Without going into a lot of details, a transponder receives the radar sweep from ATC's radar, and responds with a signal of its own. Transponders will also respond to the interrogation signal from a TAS or TCAD system. The transponder reply can contain a unique, ATC-assigned 4-digit code or the standard Visual Flight Rules code of 1200. If the transponder is set to squawk altitude, the signal will also contain the aircraft's altitude. That's right - neither ATC radar nor traffic detection systems can determine an aircraft's altitude on their own: They rely on the aircraft's transponder to tell it the altitude.
I've seen aircraft appear on a TIS or TCAD system with no altitude readout, probably the result of the pilot having the transponder turned on, but not set to squawk altitude. I've even seen this within the San Francisco mode C veil, where all aircraft are required to have an altitude-encoding transponder, to have that transponder turned on, and to be squawking altitude.
Back to my story. A few minutes after re-enabling the G1000's traffic feature, we were passing just east of Mt. Diablo when I saw a target appear on the G1000's moving map, three miles ahead and at our altitude. Then poof! The target disappeared. The pilot I was instructing saw it too and commented "That was strange." We both looked ahead, but saw nothing. After a few seconds, I heard a little voice in my head say "turn 20 degrees left." I made this suggestion aloud and the pilot turned 20 degrees left. That's when we saw the other aircraft, at our altitude, less than a half a mile away, heading directly to where we would have been had we not turned.
Following the regulations, we were east bound at 3500', an odd altitude plus 500'. The other plane was westbound and should have been at an even altitude plus 500', like 4500', but he was WAFDOF - wrong altitude for direction of flight.
Technology can be a great thing, but it's not always reliable.And if you hear a little voice in your head, you best listen to it.
Pilots should fly at an appropriate altitude for their direction of flight, but they don't always do so.
With a G1000, you have to fight the urge to stare at the pretty colors on the screen and keep scanning for traffic outside.