I have the Trafficscope VRX in my Glasair and it works great.  It has
  shown me traffic that I previously hadn't seen.  I have it wired into
  my audio system and the voice warnings are great.  It is panel
  mounted, and I have an external Comant blade antenna mounted under the
  right wing.  I can't comment on how it works sitting on the dash with
  the little antenna, as I have always had mine mounted in its current
  configuration.

Steve, Et Al

 

I am utilizing the Monroy ATD 300 set up much like Steve has done.  Comant high speed blade mounted on the bottom of the left wing about 19 feet from the same type Comant transponder antenna mounted on the bottom of the right wing.  The unit is wired into aircraft power and the audio panel for its voice warnings.  I cannot compare it to the VRX since I previously had the ATD 200.

 

I always have the 300 set on the "near" warning (1 NM) since I can't see anything smaller than a 747 much further than a mile distant (MaGoo effect).  The altitude information is quite accurate although the distance measurements are sometimes questionable and may be up to a mile off - verified by "accurate" ATC radar.  When Arnie flys alongside the zero distance indication is more accurate (just like when the fuel gauge says empty).

 

I shut the audio off (mute) when approaching a controlled airport environment since there are usually too many targets resulting in constant nagging.

 

When I previously had the antenna mounted on the hatrack, the ATD 200 was reasonably useful if I was flying solo - with a passenger along (that means two aboard) the signal must have been attenuated since certain quadrants were blanked and I saw unreported aircraft.

 

Its a big sky and the device is very useful as a second set of eyes - but no better either.  It cerainly wakes one up during a leisurely operated cross-country when the nag says "Traffic Nearby."