My experience, based on perhaps a dozen flights, is very similar. False and erratic returns are fairly common. For example I always seem to get a return when in the area of a disused airfield. At the time I would normally be talking to our local LARS and they have not reported any traffic.
On the positive side when I get a steady signal it does tend to be a positive return from traffic.
There is still a lot of traffic out there that do not or cannot use transponders like some microlights, homebuilts and most gliders.
Overall I am happy with it and would rather fly with than without it. Its an aid but not a perfect aid.