I've had 8-10 weeks experience thus far with my STi, in 3 different vehicles.
I've got to say that it doesn't false much at all, (I've been using Autoscan mode almost exclusively on my 85% rural commute).
I've also noticed that it is very depend on orientation towards the signal to get an alert. It seems to rarely see a false signal (i.e store door opener) that is coming from the side, if it's more than 2-300ft away. (perpendicular to the unit)
It's been so quiet that I've often wondered if it's malfunctioning.
For example, I could be going along past a grocery store w either a K band or X band door openner, and as long as my Sti is pointed down the road (perpendicular to the automatic door) it will stay silent 95% of the time. If I rotate the STi 45 degrees toward the door openner, I can get an alert.
This is provided that the grocery store is situated a ways back from the road (ie at the back of a med-large parking lot)
It almost seems that the STi has a very narrow field of vision. I used to get many more falses with my 1997 V1, but maybe that's cause of the dual antenna.